Which is a compound sentence? while my brother takes his dance class, i go to soccer practice. we can watch the movie after we’ve finished the book. i painted my bedroom beige, but my sister painted her room blue. they like to borrow our telescope when it’s clear enough to see the moon.

Your answer would be C, that is, the one that reads "I painted my bedroom beige, but my sister painted her room blue". Compound sentences have at least two independent clauses joined by a comma, a conjuction (as in this case) or a semicolon. Furthermore, independent clauses are understood as clauses which have a subject and a verb and can exist on their own as they form a complete thought.

The coordinating conjuctions, commas and semicolons not only join the clauses but they also create semantic relationships between them.

I painted my bedroom beige, BUT my sister painted her room blue

In this example, you can see that the subjects of the two clauses are in bold (I, my sister), the predicates are in italic (painted my bedroom beige, painted her room blue) and the coordinating conjuction joining the two clauses is in capital letters (BUT).

i think the answer is c.