Please help! Give detailed answer.

Answer:
C
Step-by-step explanation:
the figure is a rectangle with opposite equal sides
Consider the upper right triangle
using Pythagoras' identity
p² + 20² = 25²
p² + 400 = 625 ( subtract 400 from both sides )
p² = 225 ( take square root of both sides )
p = [tex]\sqrt{225}[/tex] = 15