Electrophiles are electron-deficient species that are attracted to electron-rich species, like nucleophiles, in chemical reactions. In halogenation reactions, electrophiles are responsible for accepting electrons from nucleophiles, resulting in the transfer of a halogen atom to a substrate.

Among the given options, the correct answer is option C, the halonium ion. The halonium ion is an electrophile in halogenation reactions.

Explanation:
1. Halogenation Reactions:
- Halogenation reactions involve the addition of a halogen atom (e.g., chlorine, bromine) to a substrate. These reactions can occur in the presence of a halogen source, such as a halogen gas or a halogen-containing compound.
- Electrophilic halogenation typically proceeds through a mechanism called electrophilic aromatic substitution, where an electrophile reacts with an aromatic substrate.
- In this mechanism, the electrophile attacks the aromatic ring, leading to the substitution of a hydrogen atom with a halogen atom.

2. Halonium Ion:
- The halonium ion is a positively charged species formed during the electrophilic aromatic substitution mechanism of halogenation reactions.
- It is formed by the attack of an electrophile (such as a halogen) on the aromatic ring, resulting in the formation of a three-membered cyclic intermediate.
- This intermediate contains a positively charged halogen atom, which acts as an electrophile and can accept electrons from a nucleophile.

3. Role of Halonium Ion as an Electrophile:
- The halonium ion acts as an electrophile by accepting electrons from a nucleophile, such as a negatively charged species or a lone pair of electrons on another molecule.
- The nucleophile can attack the positively charged halogen atom, leading to the transfer of the halogen to the nucleophile and the regeneration of the aromatic system.

4. Other Options:
- Nitronium ion (option A) is an electrophile in nitration reactions, not halogenation.
- Sulphonium ion (option B) is not typically involved in halogenation reactions.
- Acylium ion (option D) is an electrophile in acylation reactions, not halogenation.

In summary, among the given options, the halonium ion is the correct answer as it acts as an electrophile in halogenation reactions.