Identify the reagent used for Rosenmund reduction.

The Rosenmund reduction selectively converts acyl chlorides to aldehydes using hydrogen gas over palladium on barium sulfate ($\mathrm{H}_2 / \mathrm{Pd}, \mathrm{BaSO}_4$). The barium sulfate serves as a catalyst poison, moderating the palladium's activity to prevent over-reduction to the alcohol.

$\mathrm{H}_2 / \mathrm{Ni}$ is a standard hydrogenation catalyst, while $\mathrm{Na}-\mathrm{Hg} / \mathrm{H}_2 \mathrm{O}$ and $\mathrm{SnCl}_2-\mathrm{H}_2 \mathrm{O}$ are unrelated, being employed in other reduction mechanisms.

The correct choice is $\boxed{\text{B}}$.