Correct option (A) 42
According to Lagrange’s theorem, states that for any finite group G, the order (number of elements) of every subgroup H of G divides the order of G.
Therefore, possible subgroups of group on 84 elements are : 1, 2, 3, 4, 6, 7, 12, 14, 21, 28, 42, 84.
Subgroups with element 1 and 84 are trivial groups.
The size of a largest possible proper subgroup of G is 42.
