to find the volume of the prism, you need to know the length, height, and width.
since the length is 4 blocks wide, you can multiply 4 by the length of a single block. 4*1/2 = 4/2. do this for the height and width
Length 4/2
height 7/2
width 3/2
you would then multiply all of them together.
to multiply fractions, just multiply the numerators together and the numerators together
4/2*7/2*3/2 = 84/8
both 84 and 8 are divisible by 4 so you could simplify the fraction to 21/2
the answer is C