Rust is

Options

A. a mixture of `Fe_2O_3` and `Fe(OH)_3`
B. a mixture of FeO and `Fe(OH)_2`
C. `Fe_2O_3` only
D. a mixture of `Fe_2O_3. 3H_2O` and FeO

Explanation

**Step-by-Step Solution:**

1. **Understanding Rust**: Rust is a common term for the corrosion of iron, which occurs when iron reacts with oxygen and moisture (water) in the environment.

2. **Chemical Composition of Rust**: The primary components of rust are iron oxides and hydroxides. The most common forms include hydrated iron(III) oxide, represented as Fe2O3·3H2O, and iron(II) oxide, represented as FeO.

3. **Identifying the Correct Option**: We need to analyze the options given:
- A: Mixture of Fe2O3 and Fe(OH)3
- B: Mixture of FeO and Fe(OH)3
- C: Fe2O3 only
- D: Mixture of Fe2O3·3H2O and FeO

4. **Analyzing the Options**:
- Option A does not represent rust accurately as it includes Fe(OH)3, which is not a primary component of rust.
- Option B also does not represent rust accurately as it includes Fe(OH)3.
- Option C is incorrect because rust is not just Fe2O3; it is a combination of different compounds.
- Option D is correct because it includes both Fe2O3·3H2O (hydrated iron(III) oxide) and FeO, which are components of rust.

5. **Conclusion**: Therefore, the correct answer is option D, which states that rust is a mixture of Fe2O3·3H2O and FeO.