Concept: The question tests the advanced understanding on Overlapping sets.
Solution:
Let Hockey = H ; Football= F ; Cricket = C
We know Total = H +C+ F - [(H∩C) + (C∩H) + (H∩F)] + (H∩C∩F) + Neither
=> 50 = 20+15+11 -[ 7+4+5 ] + (H∩C∩F) + 18
=> H∩C∩F = 2
Exactly two of these sports = Only HC + Only HC + Only HF
Number of students playing ONLY Hockey and Cricket
= (H∩C)-(H∩C∩F ) = 7 - 2 = 5
Number of students playing ONLY Cricket and Football
= (C∩F) - (H∩C∩F )= 4-2 = 2
Number of students playing ONLY Hockey and Football
= (H∩F) - (H∩C∩F ) = 5-2 = 3
Thus, number of students play exactly two of these sports. = 5+2+3=10 (Option B)