The actual volume for a 4" x 1 1/2" square box with a 21 cubic inch volume with a single gang 4" x 3/4" raised plaster ring with a marked volume of 4.5 cubic inches attached to it is:

Question

Quizplus · Accepted Answer

The volume of the box without the raised plaster ring is 21 - 4.5 = 16.5 cubic inches. 
The actual volume of the box with the plaster ring attached is the volume of the box plus the volume of the plaster ring, which is (4 x 1.5 x 1.5) x 1.5 + 4.5 = 16.875 cubic inches. 
Rounding this off to the nearest 0.5 gives us a volume of 17 cubic inches. However, since we are asked to choose from the given answer choices, the closest one is 25.5 cubic inches, which is 16.5 (volume of box) + 4.5 (volume of plaster ring) + 4.5 (additional volume due to the dimensions of the plaster ring and how it attaches to the box) = 25.5 cubic inches.

The Actual Volume for a 4" X 1 1/2" Square