Transition metals:

Options

A. exhibit only diamagnetism
B. undergo inert pair effect
C. do not form alloys
D. show variable oxidation states.

Explanation

To solve the question regarding the properties of transition metals, we will evaluate each of the four statements provided and determine their validity.

### Step-by-Step Solution:

1. **Evaluate the first statement: "Transition metals exhibit only diamagnetism."**
- **Analysis**: Transition metals typically have partially filled d-orbitals, which means they contain unpaired electrons. The presence of unpaired electrons leads to paramagnetism rather than diamagnetism.
- **Conclusion**: This statement is **false**.

2. **Evaluate the second statement: "They undergo inert pair effect."**
- **Analysis**: The inert pair effect refers to the tendency of the s-electrons (ns²) in heavier elements to remain non-bonding due to their proximity to the nucleus. This effect is primarily observed in heavier p-block elements, not in transition metals.
- **Conclusion**: This statement is **false**.

3. **Evaluate the third statement: "They do not form alloys."**
- **Analysis**: Transition metals are known to form numerous alloys. This is due to their similar atomic sizes, which allows them to fit into each other's crystal lattice when melted and solidified.
- **Conclusion**: This statement is **false**.

4. **Evaluate the fourth statement: "They show variable oxidation states."**
- **Analysis**: Transition metals can exhibit variable oxidation states due to the similar energy levels of their ns and (n-1)d orbitals. Electrons can be removed from both orbitals, leading to different oxidation states.
- **Conclusion**: This statement is **true**.

### Final Summary:
- The only correct statement regarding transition metals is that they show variable oxidation states. The other three statements are false.