The following are pure substances: (a) choleric alum B. refined salt C. hydrochloric acid D. Pu.

A, choleric alum is composed of a substance, which is pure;

B, the main component of refined salt is sodium chloride, and it also contains substances such as calcium chloride, which belongs to a mixture;

C, hydrochloric acid is composed of hydrogen chloride and water, belonging to the mixture;

D the main components of calcium superphosphate are calcium phosphate and calcium dihydrogen phosphate, which belong to a mixture.

So choose a.