Amides are the least reactive carboxylic acid derivatives towards nucleophilic acyl substitution because

Athe $NH X_{2}$ group is too bulky to allow attack
Bamides have no carbonyl group
Cnitrogen is more electronegative than chlorine, making $C = O$ less polar
Dthe nitrogen lone pair is strongly delocalised into $C = O$ and $NH X_{2} X^{-}$ is a very poor leaving group

Answer: D

The

n (N) \to π^{*} (C = O)

resonance makes the carbonyl carbon much less electrophilic, and the leaving group

NH X_{2} X^{-}

(conjugate acid

NH X_{3}

, pKa about 38) is extremely basic. The other statements are false (the

NH X_{2}

group is small and the carbonyl group is present).