Concept: The question tests on basics of Set theory
Solution:
Number of people who had a vegetarian lunch = 70% of 120 = 84
Number of people who had non vegetarian = 20% of 120 = 24
Number of people who had both = 10% of 120 = 12
Total = A + B - (A ∩ B) + (A U B)'
(A U B)' = Neither = 120 - 84 - 24 + 12 = 24
Option B