On her Twitter account, Ms. Harris said it had been “upsetting” to watch a video of the exchange.

“I didn’t hear the words the man used in that moment, but if I had I would’ve stopped and corrected him,” she wrote. “I’m sorry. That word and others like it aren’t acceptable ever.”

Her team did not return a request for comment about whether questions from attendees had been screened and preapproved. The identity of the man who had asked the question was unclear.

Responses to Ms. Harris’s apology on Twitter were mixed. Some users took note of Rosa’s Law, signed by President Barack Obama in 2010, which replaced the phrase “mentally retarded individual” in federal statutes with “individual with an intellectual disability.”

The effort to change the language people use to refer to those with intellectual disabilities dates back to the 1980s, Peter Berns , the chief executive of The Arc, a national advocacy organization for people with intellectual and developmental disabilities, said on Sunday.

“It’s very upsetting to hear that phrase continued to be used so commonly in our culture today,” Mr. Berns said. He called the term “derogatory” and “deeply offensive” to people with intellectual disabilities.

“Every time it is used, it really creates a teachable moment to try to explain to members of the public why the term is so offensive and really to share the history of oppression that people with intellectual disabilities have experienced in our country over the last 100 years, if not more,” Mr. Berns said.

Mr. Berns said he was glad to see Ms. Harris apologize.

“Frankly, anyone — whether they’re a politician, a business leader, a celebrity — anyone who is using this language inappropriately” should apologize and become champions for this cause, he said.