The missing term (A) in the following equation is
$2\mathrm{F}{\mathrm{e}}^{3+}+\mathrm{S}{\mathrm{n}}^{2+}\to 2\mathrm{F}{\mathrm{e}}^{2+}+ \mathrm{A}$?

In the given reaction, the transfer of electrons is taking place between tin (Sn) and iron (Fe).

The oxidation number of the iron metal on the reaction side is +3 and the oxidation number of the iron on the product side is +2.

It means that the oxidation number of the iron decreases on the side of the product, indicating the reduction of Fe.

When iron is reduced it means that tin goes into oxidation, so the oxidation number of the tin should increase.

Now, during the reduction of 2 moles of Fe³⁺ to 2 moles of Fe²⁺, 2 electrons are gained^. Thus these 2 electrons must come from Sn²⁺.

Hence, the loss of electrons by Sn²⁺ (oxidation) is shown as-

 ${\mathrm{Sn}}^{2+}\to {\mathrm{Sn}}^{4+} +2{\mathrm{e}}^{-}$

Therefore, the complete equation is shown as-

$2\mathrm{F}{\mathrm{e}}^{3+}+\mathrm{S}{\mathrm{n}}^{2+}\to 2\mathrm{F}{\mathrm{e}}^{2+}+ {\mathrm{Sn}}^{4+}$

Hence, the correct option is A.