Element X forms a chloride with the formula XCl_2 , which is a...

The formula of the chloride is $XCl_2$, which indicates that the valency of element X is 2.

Among the given options:
Na belongs to Group 1 and has a valency of 1, forming $NaCl$.
Al belongs to Group 13 and has a valency of 3, forming $AlCl_3$.
Mg belongs to Group 2 and has a valency of 2, forming $MgCl_2$.
K belongs to Group 1 and has a valency of 1, forming $KCl$.

Since X forms a chloride with the formula $XCl_2$ and is a solid with a high melting point (characteristic of ionic chlorides of Group 2 metals), X would most likely be in the same group as Mg.

Answer: Mg