The chemical name of Cytosine is

In order to name cytosine chemically we have to move according to IUPAC nomenclature rules

The structure of cytosine is as follows

Here the parent chain is of pyrimidine since cytosine is derived from pyrimidine i.e

The figure shows pyrimidine 

Now compare it with cytosine we can see that there there is an aldehydic and amine groups attached to pyrimidine now Based on the priority order set by IUPAC aldehydes have higher priority than amine so 

Here we can see that the aldehyde group is at 2 C and the amine group is at 4 carbon which has prefixes in nomenclature as –oxo and –amine respectively

Now arranged in alphabetical order we can say that the nomenclature according to IUPAC is 4-amine -2-oxo pyrimidine.

Hence option D is the correct answer.