Which of the following are purine bases?

Purine is a heterocyclic aromatic organic compound, consists of a pyrimidine ring fused to an imidazole ring.

Therefore, (i) and (ii) are the correct options.

Pyrimidine is also a heterocyclic aromatic compound. This contains two nitrogen atoms similar to benzene. Uracil and Thymine have the following structures.

Thus, (ii) and (iv) are incorrect options.