Final answer:
Adenine has a double heterocyclic ring structure.
Step-by-step explanation:
The nitrogenous bases adenine (A) and guanine (G) have a double-ring structure with a six-carbon ring fused to a five-carbon ring and are classified as purines. The bases cytosine (C) and thymine (T) are classified as pyrimidines, which have a single carbon-nitrogen ring structure. Therefore, the base that has a double heterocyclic ring structure is adenine, option A.