Answer:
Out of your answers, it is probably A.
Explanation:
Consider this:
"The students who enjoyed the poetry class frequently gave the instructor presents."
This is correct because we do not use "frequently" to describe the students enjoying the class. It's not like they are counting how many times they are enjoying the class and then giving a gift based off of that.
The students are being described as those who enjoyed the poetry class, and then the sentence states they frequently gave the instructor presents based off of them enjoying the class.
"The students who frequently enjoyed the poetry class gave the instructor presents."
This sentence is more confusing than A, at least to me. Usually we don't say someone "frequently enjoys" something, unless we were referring to their hobbies or something, but that's unlikely. Usually, if you enjoy something, you can just say you enjoy it rather than you frequently enjoy it.
Good luck, and hope this helps you!
