Primary amines are formed when alkyl or benzyl halides are treated with excess ammonia. This reaction is known as nucleophilic substitution. Let's break down the reaction and explain why the correct answer is option 'D'.

Nucleophilic Substitution Reaction:
Nucleophilic substitution is a type of reaction where a nucleophile (electron-rich species) replaces a leaving group in a molecule. In this case, the alkyl or benzyl halide acts as the substrate and ammonia serves as the nucleophile. The reaction proceeds through a series of steps:

1. Nucleophilic attack: The lone pair of electrons on the nitrogen atom in ammonia attacks the electrophilic carbon atom of the alkyl or benzyl halide. This results in the formation of a new bond between the carbon and nitrogen atoms.

2. Leaving group departure: The halide ion is expelled as a leaving group, taking with it the electrons that were originally bonded to the carbon atom.

3. Protonation: The resulting intermediate, an alkyl or benzyl amine, is protonated by another molecule of ammonia. This step increases the stability of the intermediate and allows for further reaction.

4. Deprotonation: The protonated intermediate loses a proton, resulting in the formation of the primary amine.

Formation of Primary Amines:
Primary amines are formed when the alkyl or benzyl halide used in the reaction is a primary halide. A primary halide is one where the halogen atom is directly bonded to a carbon atom, which is in turn bonded to only one other carbon atom.

In this case, when the primary halide reacts with excess ammonia, the nucleophilic attack occurs at the electrophilic carbon atom, resulting in the formation of a primary alkyl or benzyl amine. The leaving group, in this case, is the halide ion.

Secondary and Tertiary Amines:
If the alkyl or benzyl halide used in the reaction is a secondary halide (where the halogen atom is bonded to a carbon atom, which is in turn bonded to two other carbon atoms) or a tertiary halide (where the halogen atom is bonded to a carbon atom, which is in turn bonded to three other carbon atoms), the reaction would result in the formation of secondary and tertiary amines, respectively.

However, in this specific reaction where excess ammonia is used, the primary amine is the main product formed.

In summary, treating an alkyl or benzyl halide with excess ammonia results in the formation of primary amines. The nucleophilic substitution reaction involves the attack of ammonia on the electrophilic carbon atom of the halide, followed by the departure of the halide ion and subsequent protonation and deprotonation steps.