Explanation

(b)
In the Rosenmund’s reaction the acid chlorides are converted to corresponding aldehydes by catalytic reaction. This reaction is carried in the presence of palladium deposited over barium, sulphate.
$O$
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$CH_{3} - C - Cl + H_{2}\overset{\ \ \ Pd,\ BaSO_{4}\ \ \ }{\rightarrow}CH_{3}CHO + HCl$
Acetyl chloride acetaldehyde