Answer: Ingredient C is lower.
Step-by-step explanation:
Since we have given that
Ingredient C: 1/4 cup for 2/3 serving
For
serving, number of cup = 
So, for 1 serving, the number of cups would be

Ingredient D: 1/3 cup for 3/4 serving
For
serving, number of cups = 
So, for 1 serving, the number of cups would be

So, on comparison, 
we get that Ingredient C is lower.