Ms Barrett, president Donald Trump’s pick to replace the late Ruth Bader Ginsburg as a justice on the US Supreme Court, was asked to name the five constitutional freedoms by Republican senator Ben Sasse during her confirmation hearing on Wednesday.
The senator asked: “What are the five freedoms of the First Amendment?” and the 48-year-old, replied: “Speech, religion, press, assembly.”
Ms Barrett then paused and started counting with her fingers, before she added: “Speech, religion, press, assembly... I don't know — what am I missing?”
Mr Sasse replied: “redress or protest” and Ms Barrett, who currently serves as a circuit judge on the US Court of Appeals, smiled and said: “OK.”
The senator asked the Supreme Court nominee the question while he discussed Ms Barrett teaching students about the constitution in her previous role as a law professor at the Notre Dame Law School.
The First Amendment prohibits the government from passing laws that prevent US citizens from exercising freedom of religion, speech, press, assembly, and from having the ability to petition for a redress of grievances.
The US Department of Homeland Security recommends that people applying for US citizenship study the five freedoms in preparation for their naturalisation test, according to Forbes.
Earlier in the day, Ms Barrett, a devout catholic, refused to say if she would vote to overturn decisions that provide legal protections to birth control and same-sex marriage.
Ms Barrett would not confirm whether she would have ruled with the majority in a vote that upheld legal birth control, which led Senate Judiciary Committee member Richard Blumenthal to tell her he was “stunned.”
Concerns over Ms Barrett’s strong religious beliefs have been raised before, and in 2017 Democratic senator Dianne Feinstein wondered whether she would be able to keep her beliefs separate from the court.
At the hearing, Ms Feinstein told Ms Barrett: “I think in your case, professor, when you read your speeches, the conclusion one draws is that the Dogma lives loudly within you.
“And that’s of concern when you come to big issues that large numbers of people have thought for, for years in this country.”
Ms Barrett replied that she would “follow all Supreme Court precedent without fail” and would “never impose my own personal convictions upon the law.”