In the Sandmeyer's reaction, an aryl diazonium salt is converted into various functional groups by substituting the diazonium group (-N2+) with a different atom or group. The reaction proceeds through several steps, and one of the intermediates is the diazonium salt itself.

Explanation:
1. Sandmeyer's Reaction: The Sandmeyer's reaction is a versatile method for the preparation of various functional groups, such as halides, cyanides, thiols, and phenols, by replacing the diazonium group with the desired atom or group. It involves the following steps:

a. Diazonium Salt Formation: The reaction begins with the formation of an aryl diazonium salt (Ar-N2+X-), where Ar represents the aryl group, N2+ is the diazonium group, and X- is the counterion.

b. Nitrous Acid Treatment: The diazonium salt is then treated with nitrous acid (HNO2), which results in the formation of a diazonium intermediate, which is highly unstable.

c. Diazonium Intermediate: The diazonium intermediate is formed by the loss of nitrogen gas (N2) from the diazonium salt. This intermediate is highly reactive and can undergo various substitution reactions.

d. Substitution Reaction: The diazonium intermediate then reacts with the desired reagent, which can be a halide, cyanide, thiols, or phenols. This results in the substitution of the diazonium group with the new atom or group, leading to the formation of the desired product.

2. Intermediate in Sandmeyer's Reaction: Among the given options, the diazonium salt (option A) is the intermediate in the Sandmeyer's reaction. After the formation of the diazonium salt, it undergoes further steps to generate the diazonium intermediate, which is the reactive species responsible for the substitution reactions.

3. Role of Other Options:
- Hydronium Ion (option B): Hydronium ion (H3O+) is often used as an acid catalyst in the Sandmeyer's reaction to generate nitrous acid from sodium nitrite. It does not act as an intermediate but as a catalyst.
- Ammonium Salt (option C): Ammonium salt is not involved in the Sandmeyer's reaction. It is not an intermediate.
- Nitronium Ion (option D): Nitronium ion (NO2+) is not involved in the Sandmeyer's reaction. It is commonly associated with nitration reactions and is not an intermediate in the Sandmeyer's reaction.

Therefore, the correct answer is option A, the diazonium salt, as it is an intermediate in the Sandmeyer's reaction.