In human DNA, there are nitrogenous base pairings- A, T C and G. A pairs with T and C pairs with G. Because they're a pair, the ratio of A:T and C:G should be as close to 1:1 as possible. Since A makes up approx 30.9%, T should be either C or D. As G makes up approx 19.9%, we can rule out option D, therefore the answer is C! Hope this helps (: