Which of the following statements is not correct ?

Options

A. `La(OH)_(3)` is less basic than `Lu(OH)_(3)`
B. In lanthanide series, ionic radius of `Ln^(3+)` ions decreases
C. La is actually an element of transition series rather than lanthanide series
D. Atomic radii of Zr and Hf are same because of lanthanide contraction

Answer

To solve the question "Which of the following statements is not correct?" we will analyze each statement provided in the context of d-block elements and lanthanides.

### Step-by-Step Solution:

1. **Analyze the First Statement:**
- **Statement:** Lanthanum hydroxide is less basic than dutetium hydroxide.
- **Explanation:** This statement is incorrect. Due to lanthanoid contraction, the size of lanthanum (La³⁺) is larger than that of dutetium (Du³⁺). As the size decreases, the covalency character increases, leading to a decrease in basicity. Therefore, lanthanum hydroxide is actually more basic than dutetium hydroxide.

2. **Analyze the Second Statement:**
- **Statement:** In the lanthanoid series, the ionic radius of Ln³⁺ ions decreases.
- **Explanation:** This statement is correct. As we move across the lanthanoid series from lanthanum to lutetium, the ionic radius of the Ln³⁺ ions decreases due to the increasing nuclear charge without a significant increase in shielding effect.

3. **Analyze the Third Statement:**
- **Statement:** Lanthanum is actually an element of the transition series rather than the lanthanide series.
- **Explanation:** This statement is correct. The lanthanide series starts from cerium (atomic number 58) and ends at lutetium (atomic number 71). Lanthanum (atomic number 57) is technically not part of the lanthanide series, even though it is named after it.

4. **Analyze the Fourth Statement:**
- **Statement:** The atomic radii of zirconium and hafnium are the same because of lanthanoid contraction.
- **Explanation:** This statement is correct. The lanthanoid contraction affects the size of the d-block elements, causing zirconium (Zr) and hafnium (Hf) to have similar atomic radii due to the similar electron configurations and the effects of the lanthanoid contraction.

### Conclusion:
The statement that is **not correct** is the first one: "Lanthanum hydroxide is less basic than dutetium hydroxide."

### Final Answer:
**The incorrect statement is:** Lanthanum hydroxide is less basic than dutetium hydroxide.