Which one of the following is 'd'-block element?

Options

A. Gd
B. Hs
C. Na
D. O

Explanation

To determine which of the following is a 'd'-block element, we need to understand the definition of 'd'-block elements.

### Step-by-Step Solution:

1. **Understanding 'd'-block Elements**:
- 'd'-block elements are defined as those elements where the last electron enters the d-subshell. This typically includes transition metals.

2. **Analyzing the Given Options**:
- We have four options to evaluate: GD, HS, NA, and O.

3. **Evaluating GD (Gadolinium)**:
- Gadolinium (GD) is a lanthanide and belongs to the f-block of the periodic table. Therefore, it is **not** a d-block element.

4. **Evaluating HS (Hassium)**:
- Hassium (HS) has an atomic number of 108. Its electronic configuration is [Rn] 5f14 6d6 7s2. The last electron enters the d-subshell (6d), which means Hassium is indeed a **d-block element**.

5. **Evaluating NA (Sodium)**:
- Sodium (NA) is an alkali metal with an atomic number of 11. Its electronic configuration is 1s² 2s² 2p⁶ 3s¹. Since its last electron enters the s-subshell, it is an **s-block element**.

6. **Evaluating O (Oxygen)**:
- Oxygen (O) has an atomic number of 8. Its electronic configuration is 1s² 2s² 2p⁴. The last electron enters the p-subshell, making it a **p-block element**.

7. **Conclusion**:
- Among the options provided, the only element that qualifies as a d-block element is **Hassium (HS)**.

### Final Answer:
The correct answer is **HS (Hassium)**.