How many chemical reactions are possible regarding the formation of carbonic acid and the dissolving of calcite?
a. 3
b. 2
c. 23
d. 10

I'd say B, 2 reactions. Calcite is defined as , and carbonic acid as .

The usual reaction would be to mix both components, obtaining the following:

+  = 

Which means we have 2 reactions. Nevertheless, to give you a full explanation, I'd need more information to be sure you're not talking about a reaction with other components involved.

The answer should be c.