18) C

A monoprotic acid is an acid which gives a single proton or hydrogen ion in aqueous solution.
Since magnesium is in group 2, its valency would be 2.

Therefore, the product of magnesium and propanoic acid would be Mg(C₃H₅O₂)₂.
So, we can eliminate options B, D and E.

Since the other reactant is MgCO₃ and not just Mg, carbon dioxide will also be produced as one of the products. Therefore, A is wrong.