Using a trigonometric relation we can see that the lenght of the legs is 4/√2, the correct option is B.
How to find the lengths of the legs?
Here we have a right triangle, where we know that the length of the hypotenuse is 4, and one of the interior angles measures 45° (also does the other)
So the two legs have the same lenght.
To find that lenght, we can use the relation:
sin(angle) = (opposite cathetus)/hypotenuse
replacing what we know we will get:
sin(45°) = x/4
sin(45°)*4 = x
4/√2 = x
So the correct option is B.