The number of C = O groups in thymine and X are equal. Now, X is

According to the figure given below, we can see that the given nitrogenous base, thymine, has two C = O groups. Now, if we see the number of C = O groups in the other nitrogenous bases given in the options, we can easily find out X by the given condition. Cytosine has only one, adenine has a zero and guanine has only one C = O group but uracil has two C = O groups. Thus, we can say that uracil is X as it has the same number of carbonyl groups as thymine.

Hence, the correct answer is (B) Uracil.