(A) Aldehyde are more reactive than ketone. 

(R) Nucleophilic addition reaction in carbonyl compound and $\propto$ positive charge at carbon of carbonyl group.

In contrast to ketones, aldehydes are very reactive and easily undergo nucleophilic addition reactions. Due to the electrical and steric effects, aldehydes exhibit many more favourable equilibrium constants for addition processes than ketones. Due to electronic and steric effects, aldehydes have higher favourable equilibrium constants than ketones for extra processes. 

Regarding the instance of ketones, the ketone structure has two big substituents that cause steric hindrance when the nucleophile approaches the carbonyl carbon. Aldehydes only make up one substitution, hence the steric barrier to the incoming nucleophile is relatively low. Aldehydes, on the other hand, are more reactive electrically than ketone. Ketones, as opposed to aldehydes, diminish the electrophilicity of the carbonyl carbon atoms since they have two alkyl groups.

Therefore, the correct option is option A [Both (A) and (R) are true and (R) is the correct explanation of (A)].