In the following diagram, the circle represents girls, the triangle represents science students, and the rectangle represents PG students. The numbers in different sections indicate the number of persons. How many PG students are girls who are NOT science students?

​According to the given Venn diagram:

From the above diagram 

The number 13 appears inside the circle and the rectangle but outside the triangle

​These 13 individuals are girls who are PG students but are NOT science students.

Thus, correct answer is (a)