You did the math wrong twice, yet you ended up with the right answer, (A). You were just lucky.
15,000/3000 = 5 (not 2.5)
5A*1200s = 6000 Coulombs (not 3000)
Energy/Coulombs = voltage
Your method is an acceptable way to do the problem, and gives the right answer the numbers are done right.
Here is yet another way to do it
The power is P = 15000J/1200s = 12.5 W
12.5 = V*I
V = 12.5/5 = 2.5 V