Which of the following best describes the “domino theory,” which was used as a justification for us involvement in vietnam?

You did not provide possible answers, however, the domino theory was based on the belief that if one country became a Communistic one, the others around it would too. Similar to how dominoes fall down when one falls down. That's why the US wanted to get involved so as to prevent this.

Very useful. it is to learn because history repeats itself so learning about it those mistakes not happen again.