Tu (prendre) un cours de musique.

Explanation

The correct answer is prends
The correct sentence would be: "Tu prends un cours de musique." Which translates to "You take a music course."
The subject of the sentence is "Tu" (you), which is a second person singular pronoun, so the verb "prendre" must be conjugated in the second person singular form, which is "prends".
Here's a breakdown of the other options:
prend (third person singular, "he/she/it takes")
prenez (second person plural, "you take" - formal or plural form)
prennent (third person plural, "they take")