Answer: B.

Explanation:
We can use the Pythagorean theorem where a and b are the legs and c is the hypotenuse. Then we will solve by squaring, reverse operations and isolating the variable x.
a² + b² = c²
11² + x² = 15²
121 + x² = 225
x² = 104
x =

x =

x =

This leads us to the answer of;
B.
