Final answer:
Among the options provided, cytosine and thymine are the bases with a single heterocyclic ring structure, which classifies them as pyrimidines. Therefore, the correct choice is (d) two of the above.
Step-by-step explanation:
The base that has a single heterocyclic ring structure among the options provided is cytosine. Adenine and guanine are classified as purines, which have a double ring structure, while cytosine and thymine are classified as pyrimidines, which have a single ring structure. Hence, the answer to which of the following bases has a single heterocyclic ring structure is (a) cytosine and (c) thymine, making option (d) two of the above the correct choice.