SOH = Sine Opposite Hypotenus
CAH = Cosine Adjacent Hypotenus
TOA = Tangent Opposite Adjacent
Based on the key I wrote above, TOA is what you will use. Starting from angle Z look at the opposite side and adjacent side. (Opposite = 21 and Adjacent = 20) Make sure you're not looking at the hypotenus which is the side that is always across from the 90° angle.
Going by opposite over adjacent, that would be 21/20
The answer is the first choice 21/20