Which of the following belongs to the category of transition metal?

Options

A. `K`
B. `Ra`
C. `Fe`
D. All of the above

Answer

To determine which of the given elements belongs to the category of transition metals, we need to analyze the electronic configurations of each element and see if they fall under the d-block of the periodic table.

### Step-by-Step Solution:

1. **Identify the Elements**: The elements provided are potassium (K), radium (Ra), and iron (Fe).

2. **Determine Atomic Numbers**:
- Potassium (K) has an atomic number of 19.
- Radium (Ra) has an atomic number of 88.
- Iron (Fe) has an atomic number of 26.

3. **Write the Electronic Configurations**:
- **Potassium (K)**:
- Atomic number 19: The electronic configuration is \( \text{[Ar]} 4s^1 \).
- The last electron enters the s-orbital, indicating it is an s-block element.
- **Radium (Ra)**:
- Atomic number 88: The electronic configuration is \( \text{[Rn]} 7s^2 \).
- The last electron also enters the s-orbital, indicating it is an s-block element.
- **Iron (Fe)**:
- Atomic number 26: The electronic configuration is \( \text{[Ar]} 4s^2 3d^6 \).
- The last electrons enter the d-orbitals, indicating it is a d-block element.

4. **Identify the Transition Metal**:
- Transition metals are defined as elements that have partially filled d-orbitals.
- From the configurations:
- Potassium is an s-block element.
- Radium is an s-block element.
- Iron is a d-block element with 6 electrons in the d-orbital.

5. **Conclusion**:
- The only element that qualifies as a transition metal is **iron (Fe)**.

### Final Answer:
The correct option is **iron (Fe)**.

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