The answer is d.
If you are solving for a missing leg of a triangle you want to use Pythagoras Theorm.
a² + b² = c²
a and b will be the legs you have been given and c is your hypotenuse (x).
5² + 5² = c²
25 + 25 = c²
50 = c²
since you are solving for c not c² you must take the √ of 50
√50 does not come out as an even number so you must try to think of two numbers that you can multiply and get 50 one should be a perfect square. In this case it is 25 and 2
√25 = 5
√2 does not come out even so we express our answer as 5√2
