The answer is D.
To solve it, you have to add the fractions.
1) Since it is in mixed numbers, you have to turn the fractions to improper.
2 1/2 + 1 1/4 = 5/2 + 5/4
2) Next find the lowest common multiples of the denominators( 2 and 4)
2: 2, 4
4: 4
3) Solve
5 x 2
2 x 2 = 10/4 10/4 + 5/4 = 15/4 <--make sure you only add the nominators after you find the common multiples.
4) Turn into a proper fraction
15/4 = 3 3/4
There are 3 3/4 gallons of paint all together. Therefore, answer D is correct.