The correct answer is option C) Halonium ion.

Explanation:
Halogens are electronegative elements and have a tendency to gain an electron to achieve a stable electronic configuration. In halogenation reactions, a halogen atom is introduced into an organic molecule. This reaction involves the formation of a reactive intermediate known as a halonium ion.

Halonium ion:
A halonium ion is a positively charged intermediate species that contains a three-membered ring with two carbon atoms and one halogen atom. It is formed during halogenation reactions when a halogen molecule reacts with an alkene or alkyl group.

Reactivity of halonium ion:
The halonium ion acts as an electrophile, which means it is an electron-deficient species that can accept a pair of electrons from a nucleophile. The positively charged halogen atom in the halonium ion is attracted to the electron-rich region of the nucleophile, leading to the formation of a new bond.

Reasons why the other options are not the correct answer:

a) Nitronium ion:
The nitronium ion (NO2+) is a strong electrophile and is involved in nitration reactions. It is formed by the reaction of nitric acid with a strong acid. However, it is not directly involved in halogenation reactions.

b) Sulphonium ion:
Sulphonium ions are positively charged species that contain a sulfur atom bonded to three organic groups and a positive charge. They are commonly used as strong alkylating agents and are not directly involved in halogenation reactions.

d) Acylium ion:
Acylium ions are formed by the removal of a leaving group (usually a halide) from an acyl halide or other acyl derivatives. They are electron-deficient species and are involved in acylation reactions, not halogenation reactions.

In conclusion, the correct answer is option C) Halonium ion. It acts as an electrophile in halogenation reactions, where a halogen atom is introduced into an organic molecule.