The electrons which take part in order to exhibit variable oxidation states by transition metals are:

Options

A. ns only
B. (n-1)d only
C. ns and (n-1)d only but not np
D. (n-1)d and np only but not ns

Answer

To solve the question regarding which electrons participate in exhibiting variable oxidation states by transition metals, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Transition Metals**:
Transition metals are defined as elements that have partially filled d-orbitals. These d-orbitals play a crucial role in the variable oxidation states exhibited by these metals.

**Hint**: Remember that the presence of partially filled d-orbitals is key to understanding the variable oxidation states.

2. **Identifying Electrons Involved**:
The electrons involved in oxidation states are typically from the outermost shell (ns) and the d-orbitals from the penultimate shell (n-1)d. For example, in the case of iron (Fe), which has an atomic number of 26, its electron configuration is:
\[ \text{Fe: } 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^6 \]
Here, the 4s and 3d electrons are the relevant ones.

**Hint**: Focus on the outermost (ns) and the d-orbitals from the previous energy level (n-1)d.

3. **Analyzing the Options**:
The options provided are:
- (i) ns only
- (ii) n-1 d only
- (iii) ns and n-1 d only but not np
- (iv) n-1 d and np but not ns

From our understanding, we know that both ns and (n-1)d electrons contribute to variable oxidation states, while np electrons do not significantly participate since they are usually filled.

**Hint**: Compare the options with your understanding of which orbitals are involved in variable oxidation states.

4. **Selecting the Correct Option**:
Based on the analysis, the correct option is (iii) ns and n-1 d only but not np. This is because the np orbitals are filled and do not contribute to variable oxidation states.

**Hint**: Eliminate options that do not include both ns and (n-1)d.

### Final Answer:
The electrons which take part in order to exhibit variable oxidation states by transition metals are **ns and (n-1)d only but not np** (Option iii).