Explanation:
Salt is formed when a base reacts with an acid. This is known as a neutralization reaction. During a neutralization reaction, the hydrogen ions from the acid react with the hydroxide ions from the base to form water. At the same time, the cations from the base combine with the anions from the acid to form a salt.
Here is a detailed explanation of each option and why it is incorrect or correct:
A: Metals react with water - This is incorrect because when metals react with water, they typically form metal oxides or metal hydroxides, not salt.
B: Metals react with oxygen - This is incorrect because when metals react with oxygen, they typically form metal oxides, not salt.
C: Base reacts with acid - This is correct. When a base reacts with an acid, a neutralization reaction occurs and salt is formed.
D: None of these - This is incorrect because the correct answer is C, as explained above.
To summarize, the correct answer is C: base reacts with acid.

Salt is formed when a base reacts with an acid. This reaction is known as a neutralization reaction, where an acid and a base combine to form a salt and water. Let's explore this process in detail.

1. Definition of Salt:
Salt is a compound formed when the hydrogen ions (H+) from an acid are replaced by metal or ammonium ions. It is an ionic compound composed of positively charged ions (cations) and negatively charged ions (anions). The cation comes from a metal or ammonium, while the anion is derived from an acid.

2. Acid-Base Reaction:
An acid is a substance that donates hydrogen ions (H+) when dissolved in water, while a base is a substance that donates hydroxide ions (OH-) when dissolved in water. When an acid and a base react, the hydrogen ions from the acid combine with the hydroxide ions from the base to form water (H2O).

3. Formation of Salt:
During the acid-base reaction, the remaining ions combine to form a salt. The cation in the salt comes from the base, while the anion comes from the acid. For example, when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), sodium chloride (NaCl) is formed. The hydrogen ions from hydrochloric acid combine with the hydroxide ions from sodium hydroxide to form water, and the remaining sodium and chloride ions combine to form sodium chloride (salt).

4. Examples:
Here are a few common examples of salt formation through acid-base reactions:
- Hydrochloric acid (HCl) + Sodium hydroxide (NaOH) → Sodium chloride (NaCl) + Water (H2O)
- Sulfuric acid (H2SO4) + Calcium hydroxide (Ca(OH)2) → Calcium sulfate (CaSO4) + Water (H2O)
- Nitric acid (HNO3) + Potassium hydroxide (KOH) → Potassium nitrate (KNO3) + Water (H2O)

In all these reactions, an acid reacts with a base to form a salt and water. The salt formed is a result of the combination of the remaining ions from the acid and base.

To summarize, salt is formed when a base reacts with an acid in an acid-base reaction. The hydrogen ions from the acid combine with the hydroxide ions from the base to form water, while the remaining ions combine to form a salt. This process is known as neutralization, and it is a fundamental concept in chemistry.