An atom with atomic number 21 belongs to the category of:(A) s- block elements(B) p-block elements(C) d- block elements(D) f-block elements

The atomic number 21 corresponds to the element **Scandium (Sc)**. Now, let's determine which category it belongs to based on its electronic configuration.

1. **Atomic number (Z) = 21**
2. **Electronic configuration of Scandium:**
\[
\text{Sc} (Z = 21): [Ar] \, 3d^1 4s^2
\]

3. **Category Determination:**
- **s-block elements:** These are elements where the last electron enters an **s orbital**. However, for Scandium, the last electron enters the **3d orbital**.
- **p-block elements:** These are elements where the last electron enters a **p orbital**. Scandium does not have its last electron in the p orbital.
- **d-block elements:** These are elements where the last electron enters a **d orbital**. In the case of Scandium, the last electron enters the 3d orbital, making it a **d-block element**.
- **f-block elements:** These are elements where the last electron enters an **f orbital**, which is not the case for Scandium.

Since the last electron enters the **3d orbital**, **Scandium (atomic number 21)** belongs to the **d-block** of the periodic table.

### Answer:
The correct option is **(C) d-block elements**.