Let's use the Double Matrix Method. This technique can be used for most questions featuring a population in which each member has two characteristics associated with it.
Here, we have a population of students, and the two characteristics are:
- taking French or not taking French
- taking Spanish or not taking Spanish

From our given information, we can set up our matrix as follows:


Since there are 50 students altogether, we know that, if 31 are taking French then 19 are NOT taking French
And, if 17 are taking Spanish then 33 are NOT taking Spanish


When we fill in the rest of the matrix we get:


So, there are 8 students taking BOTH French and Spanish

Answer: B

This question type is VERY COMMON on the GRE, so be sure to master the technique.

To learn more about the Double Matrix Method, watch this video: