4. Option B is correct
Given the chemical equation below;
[tex]Ag+H_2S\rightarrow Ag_2S+H_2O[/tex]Chemical equation is known to be balanced if the total atom of elements are on both the reactant and product are equal.
[tex]4Ag+2H_2S+O_2\rightarrow2Ag_2S+2H_2O[/tex]From the balanced chemical equation, the coefficient for Ag is 4