b
Stoichiometry deals with measurements of reactants and products in a chemical reaction.

\(a A(g)+b B(g) \rightarrow c C(g)+d D(g)\)

Here, '\(a\)' moles of \(A(g)\) reacts with '\(b\)' moles of \(B(g)\) to give '\(c\)' mole of \(C(g)\) and '\(d\)' moles of \(D(g)\)

No. of moles of \(\mathrm{AgNO}_{3}=10^{-3} \mathrm{mol}\)

No. of moles the chloride \(=0.5 \times 10^{-3} \mathrm{mol}\)

Suppose the formula for the chloride is \(\mathrm{XCl}_{\mathrm{n}}\) then moles of chloride ion \(=\mathrm{n} \times 0.5 \times 10^{-3}\)

Reaction goes as follows:

\(A g^{+}+C l^{-} \rightarrow A g C l\)

Then, going by stoichiometry we get

\(n \times 0.5 \times 10^{-3}=10^{-3}\)

\(\Rightarrow n=2\)

Therefore, formula is \(\mathrm{XCl}_{2}\)