We know –
Order of an element of a group = Order of subgroup generated by that element.
Subgroup generated by an element "a" of a group = Subgroup generated by "$a^{-1}$".
Now, Order of a = 6 --- given.
Thus, $a^6$ = e (identity element).
Left multiplying by $a^{-1}$ on both sides.
Thus, a$^5$ = $a^{-1}$.
Thus, subgroup generated by a = subgroup generated by $a^5$.
Thus, Order of a = Order of $a^5$ = 6.
Answer :- C. 6