Answer: tan x
Step-by-step explanation: An angle determines if a side is opposite the angle, or adjacent the angle. Since p/a is something to do with the tangent, as it is not using the hypotenuse (the longest side.) Tangent is Opposite/Adjacent, while Sine is Opposite/Hypotenuse and Cosine is Adjacent/Hypotenuse. The only thing that makes sense is tangent.
So, that leaves us with either tan y or tan x. Well, with x, the angle that x points to (which is the opposite side) is p, and the adjacent is a, so p/a must be Tangent(x) since it is Opposite/Adjacent sides.
Only if it was r/p would it be tan y, since r becomes the opposite and p becomes the adjacent.