One recipe calls for 3/4 cup of milk. a different recipe calls for 1/2 as much milk as the first recipe. how much milk does the second recipe call for?

For this problem you need to divide 3/4 by 2. For division of fractions, invert and multiply. 

2/1 = 1/2

3/4 • 1/2

3 • 1 = 3
4 • 2 = 8

1/2 of 3/4 cup of milk = 3/8 of a cup of milk.

Cbottom left is the correct one.