<u>Answer:</u> The correct answer is Option 2.
<u>Explanation:</u>
For the given options:
<u>Option 1:</u> Hydroxide ion
The chemical formula for this ion is 
<u>Option 2:</u> Hydronium ion
The chemical formula for this ion is 
<u>Option 3:</u> Hypochlorite ion
The chemical formula for this ion is 
<u>Option 4:</u> Perchlorate ion
The chemical formula for this ion is 
For the given chemical reaction:

The ion represented by X is hydronium ion.
Hence, the correct answer is Option 2.