(A) $\mathrm{RCOCl}$ fails to give ketone when treated with $MgX$ but with $R_{2}Cd$  ketones are obtained easily. 

(R)   $\mathrm{R}-\mathrm{Cd}$ bond is less polar as compared to $\mathrm{R}-\mathrm{Mg}$ bond.

Acid chloride is treated with Grignard’s reagent it fails to give ketone but in case of Di alkyl cadmium it gives ketone. The bond polarity for $Cd$ is less compared to$Mg$ Hence, ketone is easily obtained with Di alkyl cadmium not with$Mg$ where polarity is higher hence reaction does not takes place. 

Therefore, both the statement (A) and (R) are true and (R) is the correct explanation of (A)

Hence, Option A is correct