first step is to find the area of the triangle on the end:
area of a triangle = 1/2 x base x height:
base = 4m and height = 2m
area = 1/2 x 4 x 2 = 4 square meters
now for volume multiply the are of the triangle by the depth which is 5m
volume = 4 x 5 = 20 cubic meters
the answer is D