Question

highlight the word, clause, or phrase that modifies the word(s) in bold.
captain marvel, whom i facebook message every day, is dressed as a clown in her profile picture.

Explanation:

In the sentence, the bolded word is "Captain Marvel". The relative clause "whom I Facebook message every day" is a non - restrictive relative clause that provides additional information about "Captain Marvel", thus modifying it.

Answer:

The phrase (clause) that modifies "Captain Marvel" is "whom I Facebook message every day"