Let C's share = x. C acquires 25% of his share from A, so 0.25x from A and the remainder from B. B surrenders 1/4 of the firm's share, i.e., 1/4 to C, which is 0.75x. Hence 0.75x = 1/4, giving x = 1/3.
Note: The NTA wording is ambiguous; we interpreted B's surrender as 1/4 share of the firm (the only interpretation consistent with the options).