Which is the best estimate for the sum of 3/8 and 1/12?

The best estimate is 11/24.

To solve, find a common denominator between both 8 and 12. They can both multiply into 24. 

To turn 8 into 24, you multiply by 3, so you must do the same with its numerator. To turn 12 into 24, you multiply by 2, so again, you must do the same with its numerator:

3/8 ---> 9/24
1/12 ---> 2/24

Then, you can add the numerators together and keep the denominator 24: 

9/24 + 2/24 = 11/24.