Identify the catalyst used in following reaction at $500^{\circ} \mathrm{C}$ ? $\mathrm{CO}+\mathrm{H}_2 \mathrm{O} \rightleftharpoons \mathrm{CO}_2+\mathrm{H}_2$

The water-gas shift reaction is $\mathrm{CO}(\mathrm{g}) + \mathrm{H}_2\mathrm{O}(\mathrm{g}) \rightleftharpoons \mathrm{CO}_2(\mathrm{g}) + \mathrm{H}_2(\mathrm{g})$, used to produce hydrogen gas from carbon monoxide and steam. Catalyst selection depends on operating temperature.

At high temperatures around $500^\circ \mathrm{C}$, the catalyst is an iron-chromium oxide mixture, denoted $\mathrm{Fe}-\mathrm{Cr}$. Iron oxide $\mathrm{Fe}_2\mathrm{O}_3$ serves as the primary catalyst, while chromium oxide $\mathrm{Cr}_2\mathrm{O}_3$ acts as a stabilizer to prevent sintering and maintain activity.

Evaluating the options: $\mathrm{Fe}-\mathrm{Cr}$ is correct for this temperature range. Nickel catalysts are used for hydrogenation, cobalt-thorium for Fischer-Tropsch synthesis, and platinized asbestos for oxidation or hydrogenation—none are standard for high-temperature water-gas shift.

The catalyst is therefore $\boxed{\text{A}}$.