Final answer:
Cytosine pairs with the base guanine through complementary base pairing in nucleic acids.
Step-by-step explanation:
According to the rules of complementary base pairing in nucleic acids, cytosine pairs with the base guanine.
In DNA, adenine pairs with thymine, and cytosine pairs with guanine.
These base pairs are stabilized by hydrogen bonds. Adenine and thymine form two hydrogen bonds, while cytosine and guanine form three hydrogen bonds between them.