My daughter Whitney put me on the spot during a chat we had about finances while driving back to college for her senior year.

I had suggested, among other things, that she open a Roth IRA to get a leg up on saving for retirement that will dramatically improve the rest of her financial life.

Whitney liked the sound of the idea. She’s financially savvy and has worked hard to make and manage money, but she has never, to this point, owned a mutual fund.

“So you are telling me to buy a mutual fund that I would want to invest in for the next 50 years.,” she said. “Okay, what fund should I buy?”

It’s both a simple query and an impossibly difficult proposition.

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In the early days of fund investing — effectively from the 1920s until the mid-1980s — an investor would have simply picked a managed fund, where the manager could go anywhere with the money, providing professional management and diversification at a reasonable price.

Today, it’s hard to find a great go-anywhere fund, especially if you need to believe management can deliver above-average results for longer than an entire career and beyond.

When I started covering the fund industry around the time Whitney was born, standard advice would have been to buy a balanced fund for its professionally managed mix of stocks and bonds. It makes sense as a first fund, but it’s not a great building block for a portfolio once an investor wants to take control of their asset allocation.

The alternative would have been a stock-market index fund, for its low-cost approach to a market that is likely to be up over any period of decades-long investing.

Today, however, there are so-called smart-beta indexes — benchmarks that rebalance automatically or that reflect the S&P 500 SPX, -1.15% in equal-weighting instead of by market value, for example — and target-date funds that mix funds to achieve a portfolio built to last a lifetime and to change as the investor ages.

In short, picking one fund top hold for the next half-century is a tall order.

Warren Buffett is recommending a portfolio for his wife — assuming she survives him — that is 90% invested in the Vanguard 500 Index VFINX, -1.15% ; that kind of long-term bet on the U.S. and its biggest corporate names, made at ultra-low costs, was my first instinct.

Ask experts, as I did, and their gut feeling will tend towards what has worked for them and their families, or to choices in line with risk tolerances.

For example, Dan Wiener, editor of The Independent Adviser for Vanguard Investors, told his children to buy PrimeCap Odyssey Aggressive Growth POAGX, -1.82% — his long-time personal favorite — and would today advise buying PrimeCap Odyssey Growth POGRX, -2.02% because his first choice is closed to new investors.

Similarly, Jason Browne, chief investment officer at the Fund*X Investment Group, went with the FundX Upgrader fund FUNDX, +0.04% for his daughter’s first Roth IRA, choosing the house strategy because it adapts to changing market leadership over time, allowing it to stay current even as trends change over the next few decades.

Jeff Tjornehoj, head of Americas Research for Lipper Inc., said he’d “go aggressive … 100% emerging markets. As she accumulates wealth she can add less-aggressive strategies to diversify.”

While there are merits to that riskier approach, it also has the potential to turn out like buying a Japan fund in the mid-1980s, when I was starting out. For all of the potential of emerging markets, making it the focus of a one-fund portfolio seems too aggressive.

Mark Salzinger, editor of the No-Load Fund Investor newsletter, went the conservative route with the target-date funds, noting his preference in that genre is for issues run by T. Rowe Price. He pointed out that T. Rowe Price Retirement 2060 TRRLX, -1.53% is roughly 85% equities, but includes foreign stocks and a mix of both index- and actively managed funds, effectively achieving instant diversification and lifetime management.

Any or all of those paths could work for Whitney.

She seemed torn between indexing and target-date investing, acknowledging that the more she learns and invests over time, the less her first fund will feel like a big deal and the more it will be just another tool in the retirement-savings kit.

Ultimately, that brought me to some advice that Steven Lipper of the Royce funds gave to his children, and first heard from his father, Michael Lipper, the founder of Lipper Inc., which is that the actual first fund (or funds) matters less than the engagement of the owner.

Whatever Whitney lands on will be a fine choice, provided she is a good fund owner, keeping a watchful, thoughtful eye on her investments.

“Few things grow over the decades without attention,” Steven Lipper said. “Understanding that your money needs care, attention and nurturing in the way that your health, your relationships and other important components of your life gets our young adults started with a helpful mindset. Moreover, when you show attention and respect for your money you show respect to its source — your own work or someone who gave it to you.”