To help you with this in the future, look at all the terms/variables to find matches. For example, you were given 12ad^3 so look at all the answers and see if you find a number next to ad and raised to the third power. The only answer that matches with 12ad^3 is 3ad^3. Always make sure the variables (letters) match up in their proper positions. Do you see how the variables match up with 12ad^3 and 3ad^3? They're in the EXACT same positions.
Good luck,