Assertion (A): Following is a balanced chemical equation for the action of steam on iron: $3\text{Fe} + 4\text{H}_2\text{O} \longrightarrow \text{Fe}_3\text{O}_4 + 4\text{H}_2$.

Reason (R): The law of conservation of mass holds good for a chemical equation.

The assertion states that $3\text{Fe} + 4\text{H}_2\text{O} \longrightarrow \text{Fe}_3\text{O}_4 + 4\text{H}_2$ is a balanced chemical equation for the action of steam on iron. Counting the atoms on both sides: LHS has $3\text{ Fe}$, $8\text{ H}$, and $4\text{ O}$ atoms; RHS has $3\text{ Fe}$, $8\text{ H}$, and $4\text{ O}$ atoms. Thus, the equation is balanced, making the assertion true. The reason states that the law of conservation of mass holds good for a chemical equation, which is also true because mass can neither be created nor destroyed in a chemical reaction. Since the law of conservation of mass is the fundamental reason why we must balance a skeletal chemical equation (to ensure the number of atoms of each element remains the same before and after the reaction), the reason correctly explains why the given equation is balanced. Therefore, option (a) is correct.