Javier and Kaitlyn enjoyed playing YOUR piano today.
Answer is letter B.
YOUR is the second person possessive adjective used to describe something that belongs to YOU. Its usage implies ownership; meaning you own the piano that Javier and Kaitlyn is playing. YOUR is always followed by a noun or a gerund. In this case, piano - a noun.
You're is the contraction of "you are" and is often followed by the present principle (verb form ending in -ing.)
