Final answer:
In DNA, the nucleotide guanine pairs with cytosine, forming a complementary base pair held together by three hydrogen bonds. The correct answer is c).
Step-by-step explanation:
In a DNA molecule, guanine pairs with cytosine. Guanine is one of the four nitrogenous bases found in DNA and is a double-ringed structure known as a purine. Cytosine, on the other hand, is a single-ringed structure called a pyrimidine. The base pairing rules dictate that each base pair consists of one purine and one pyrimidine. Thus, the pair for guanine is cytosine, and these two bases form complementary base pairs that are stabilized by three hydrogen bonds. The correct answer to which nucleotide pairs with guanine in a DNA molecule is c) Cytosine.