The general chemical equation for a displacement reaction can be written as:
 A + Salt solution of B → Salt solution of A + B

The elements that A and B respectively represent are

In a single displacement reaction, a more reactive element displaces a less reactive element from the compound of the latter.

Magnesium is more reactive than zinc. Thus, magnesium can displace zinc from the salt solution of zinc to produce a salt solution of magnesium and metal zinc. The general equation used to represent the reaction can be written as:

Mg + Salt solution of Zn → Salt solution of Mg + Zn

Therefore, elements A and B likely represent Magnesium and Zinc respectively.