Which nitrogen base most closely resembles cytosine and why?

The nitrogenous base that most closely resembles cytosine is uracil. This is because they are both pyrimidines; another possible answer would be thymine, which is also a pyrimidine. On the other side of the spectrum we have adenine and guanine, which are both purines. 

If you look at the chemical structures of thymine, uracil, and cytosine side-by-side, you will see that all of them have one carbon ring (in contrast to purines, which have two rings). Uracil and Cytosine are almost completely the same structurally except that uracil has an O group while cytosine has NH2 in that respective place. (Refer to picture below)