I would have used none of them. To be exact, my usual translation of “to attend a class” is “assister à un cours”.

You could also use the word suivre.

Twice a week she attends French classes. Deux fois par semaines elle suit des cours de français.

In the context of an active attendance, participer is suitable.

I will attend this meeting. Je participerai à cette réunion.

To follow up on the 3 “correct” answers, there are some differences in the 3 proposed translations

• assister à un cours : you are going to attend the class at a specific date. It not mean that you are attending the class for a whole semester.

• suivre un cours : here you are attending the class which lasts for several sessions

• participer à un cours / prendre part à un cours : as @Shlublu noted, active attendance is implied. Nothing implied about the number of sessions.