To find out how much fabric Sadie will need to make 5 costumes, we can multiply the amount of fabric needed for one costume by the number of costumes:
\[ 3.75 \text{ yards} \times 5 = 18.75 \text{ yards} \]
So, the amount of fabric Sadie will need to make 5 costumes is 18.75 yards.
The correct answer is D. 18.75 yards.