Corrosion may be defined as the process of slow eating up of a metal by the gases and water vapours present in air due to formation of certain chemical compounds.
Corrosion is favoured by the following factors:
(i) Position of metal in the reactivity series: Active metals placed above hydrogen in the reactivity series are easily corroded as compared to the metals which are placed below hydrogen.
(ii) Air and moisture: The presence of water vapours and gases like `CO_(2), SO_(2)` etc. in air helps the process of corrosion.
(iii) Uneven metal surface: If the surface of metal is uneven, it will have certain depressions. Water drops will stick in these and take part in the chemical process leading to corrosion.
(iv) Presence of salts: Presence of salts or electrolytes in water promotes corrosion. For example, rusting of iron is faster in sea water (also called saline water) than in ordinary water or distilled water.
Chemically rust is

Options

A. hydrated ferrous oxide
B. hydrated ferric oxide
C. only ferric oxide
D. none of these.

Explanation

### Step-by-Step Solution:

1. **Understanding Corrosion**: Corrosion is defined as the gradual destruction of metals due to chemical reactions with environmental factors such as air and moisture. This process leads to the formation of various chemical compounds.

2. **Factors Favoring Corrosion**:
- **Reactivity Series**: Metals that are more reactive (those above hydrogen in the reactivity series) tend to corrode more easily than less reactive metals.
- **Air and Moisture**: The presence of water vapor and gases like carbon dioxide (CO₂) and sulfur dioxide (SO₂) in the air accelerates the corrosion process.
- **Uneven Metal Surface**: An uneven surface can trap water droplets, which can lead to localized corrosion.
...