Answer:
The answer is C. Guanine
Explanation:
Pyrimidines always bond with purines. Since Cytosine is a pyrimidine, it must bind with a purine, which can either be Adenine or Guanine. However, Adenine has only 2 places for hydrogen bonding while Guanine has 3. Since Cytosine also has 3, it would bind to Guanine. GCAT is a useful acronym for remembering which binds to which.
