An aqueous solution is known to contain Ag+, Mg2+, and Sr2+ ions....

Which reagent should be used to selectively precipitate the Ag+?
The answer is 0.2 M NaCl

I need to know why it is what it is
Please answer this question ASAP

in solution the NaCL will form ions of Na+ and CL-

The Cl- will form ionic bond with Ag+ to form AgCl

Mg2+ and Sr2+ wil need two chlorines to form stable substance, thus AgCL will be formed in preferance.