When sodium hydroxide (NaOH) reacts with benzoic acid (C6H5COOH), the product formed is sodium benzoate (C6H5COONa). This reaction occurs through the process of neutralization.

Here's a detailed explanation of why the correct answer is option 'C' (benzoic acid):

1. Introduction:
- Sodium hydroxide (NaOH) is a strong base, while benzoic acid (C6H5COOH) is a weak acid.
- When a strong base reacts with a weak acid, a neutralization reaction occurs.

2. Neutralization Reaction:
- In the neutralization reaction between NaOH and benzoic acid, the H+ ion from the acid reacts with the OH- ion from the base to form water (H2O).
- The remaining ions, Na+ and C6H5COO-, combine to form the salt sodium benzoate (C6H5COONa).
- The balanced equation for the reaction is as follows:
C6H5COOH + NaOH → C6H5COONa + H2O

3. Explanation of the Correct Answer:
- Option 'C' (benzoic acid) is the correct answer because it is the only compound among the given options that is an acid capable of reacting with NaOH to form sodium benzoate.
- Let's analyze the other options to understand why they are not the correct answers:

a) Benzene:
- Benzene (C6H6) is a hydrocarbon and does not contain any acidic or basic functional groups.
- It cannot undergo a neutralization reaction with NaOH.

b) Benzene hydroxide:
- Benzene hydroxide does not exist as a stable compound.
- It is a highly reactive intermediate formed during certain chemical reactions but cannot be isolated or used directly in a reaction.

d) Benzaldehyde:
- Benzaldehyde (C6H5CHO) is an aldehyde, which is a functional group containing a carbonyl group (-CHO).
- While aldehydes can undergo various reactions, they do not react with NaOH to form sodium benzoate.

4. Conclusion:
- Among the options given, only benzoic acid (option 'C') can undergo a neutralization reaction with NaOH to form sodium benzoate.
- By reacting with NaOH, benzoic acid loses its acidic properties and is converted into its sodium salt, sodium benzoate.