DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid) are both polynucleotides.
They are made up of monomers called nucleotides, which are in turn made up of a nitrogenous base, a pentose sugar and a phosphate group.
The nitrogenous bases in DNA and RNA are of two types: Purines and Pyrimidines.
Adenine and Guanine are Purines. Cytosine, Uracil, and Thymine are Pyrimidines.
The purine bases are common in both DNA and RNA. But DNA consists of Thymine and Cytosine as pyrimidine bases and RNA contains Uracil and Cytosine as the pyrimidine bases.
Therefore, the common pyrimidine in both DNA and RNA is Cytosine.