Answer:
B. 6sqrt(2). 
Explanation:
Since the two legs of the right triangle are congruent, this is a 45-45-90 triangle. That means that the hypotenuse will measure xsqrt(2) units, and each leg will measure x units. 
In this case, x = 6. 
So, the hypotenuse is B. 6sqrt(2). 
Hope this helps!