WASHINGTON — Transportation Secretary Elaine Chao failed last year to cash out her stock options in one of the nation’s largest suppliers of highway construction materials, despite a promise she had made to do so in a signed ethics agreement when she joined the Trump administration.

Ms. Chao had served for about two years on the board of directors of the company, Vulcan Materials, an Alabama-based supplier of rock aggregate, which is used in road construction and many other building projects. The board position paid Ms. Chao $110,000 plus $151,000 in stock options in 2016, according to a filing by the company.

As part of her ethics agreement, Ms Chao said that by April 2018 she would take “a cash payout for all of my vested deferred stock units” from Vulcan, effectively ending her financial relationship with the company.

But a financial disclosure report released this month by her husband, Mitch McConnell, the Kentucky Republican who is the Senate majority leader, showed that Ms. Chao had somewhere between $250,000 and $500,000 worth of Vulcan stock. She owned this stock because in April 2018 Vulcan paid her for her stock options in the company’s stock instead of cash, the company said in a statement. Details of her continued ownership of Vulcan stock were reported on Tuesday by The Wall Street Journal.