Trick question!

An episode is

Incorrect answer A. the beginning of a sonata.

Incorrect answer B. a scene from an opera.

Incorrect answer C. a key change.

Correct answer D. a section of a rondo.

This is a question asking about a vocabulary term in music composition. An episode in music composition is a section of a rondo.

Yes you're right ... The correct answer is Choice D

Although Choice C may look like the correct answer because e ach episode is normally in a different key from the main key. Yet, a key change is not what an Episode is; an Episode is a section of a Rondo that involves a key change.

In rondo form, the refrain (which forms the principal theme) alternates with the episodes (which may be one or more contrasting themes). Other names for Episodes are "digressions" or "couplets".