Most useful reagent for gravimetric analysis of silver My question reads as follows: 

"What's the most useful reagent for the gravimetric analysis of silver from the options below:
A. Sodium nitrate
B. Barium carbonate
C. Potassium sulphate
D. Ammonium chloride"

I can rule out A and C as they do not form a silver precipitate with a solution containing silver. However, both B and D can form a silver precipitate so I am not sure which of them would be the most useful.
You are missing something: before you even put the silver in, NH4Cl is soluble, while BaCO3 has already precipitated (insoluble)!

This means that Cl- will precipitate more completely with NH4Cl.  Therefore, D.