There are a few issues with the wording of this question.
First of all, the GMAT will not expect test-takers to know what HCF stands for.
There's also some ambiguity about whether the number pair (45, 60) is different from (60, 45)
Here's how I would have written the question...

How many unique pairs of integers (x, y) exist such that 40 < x < y < 100, and the greatest common divisor of x and y is 15?
A 3
B 4
C 5
D 6
E 7

If 15 is the greatest common divisor of x and y, then 15 must be a divisor of x and y
Or we can say that x and y must be multiples of 15

There are only four multiples of 15 in the given range: 45, 60, 75, 90
Since the answer choices range from 3 to 7, let's just LIST the possible pairs where the greatest common divisor is 15
(45, 60)
(45, 75)
(60, 75)
(75, 90)

There are 4 such pairs

Answer: B

Cheers,
Brent