Solutions are always formed when:

a) one substance dissolves uniformly in another substance

b) liquids are mixed together

c) a compound is created

d) solids and liquids are mixed together

The answer would be: a) one substance dissolves uniformly in another substance

The solution can be made from solid, liquid or even gas as long as they are dissolves uniformly in the solvent.

It's not mandatory that a solution is made from solid mixed in liquid. If a compound made that means there is a chemical reaction happens, and that is not related to the solution.