Answer:
Option D.

Explanation:
we know that
In the right triangle ABC
The function cosine of angle C is equal to divide the adjacent side to the angle C (AC) by the hypotenuse (BC)
substitute the values
cos(C)=15/20 ----> equation A
In the right triangle CAD
The function cosine of angle C is equal to divide the adjacent side to the angle C (CD) by the hypotenuse (AC)
substitute the values
cos(C)=x/15 -----> equation B
equate equation A and equation B

Simplify
Divide by 5 both numerator and denominator
