answer is C. 10
triangle is a right triangle
so you can use the Pythagorian theorem
and
2 out of the 3 sides are equal
so 2 sides are 5√2
5√2 = √5*5*2 = √50
Pythagorian theorem is
a^2 + b^2 = c^2
so
√50^2 + √50^2 = c^2
50 + 50 = c^2
100 = c^2
√100 = c
c = 10