The elements in which the last electron enter the ante-penultimate energy level, i.e., (n - 2)f-orbital are called f-block elements. These elements have been termed as f-block elements as the last electron enters in one of the f-orbitals. These elements are also known as the inner transition elements. This is because the last electron in them enters into (n - 2)-orbital, i.e., inner to the penultimate energy level and they form a transition series within the transition series. The general electronic configuration is:

\[(n - 2)f^{1-14}(n - 1)d^{0-1}ns^2\]

Classification of f-block elements:

Depending upon whether the last electron enters a 4f-orbital or a 5f-orbital, the f-block elements have been divided into two series as follows: 

(i) Lanthanides:The elements in which the last electron enters one of the 4f-orbitals are called 4f-block elements or first inner transition series. These are also called lanthanides or lanthanons or lanthanoids because they come immediately after lanthanum.

(ii) Actinides: The elements in which the last electron enters one of the 5f-orbitals are called 5f-block elements or second inner transition series. These are also called actinides or actinons or actinoids because they come immediately after actinium.

The lanthanoid contraction is responsible for the fact that

Zr and Hf have almost the same radius

The correct answer is option 3. Zr and Hf have almost the same radius.

The lanthanoid contraction phenomenon primarily affects the atomic and ionic radii of transition metals. Let's analyze each option in light of the lanthanoid contraction:

1. Zr and Y have almost the same radius:

This statement is incorrect. Yttrium (Y) is not a lanthanoid element, so the lanthanoid contraction does not directly impact its size compared to Zirconium (Zr). The similarity in size between Zr and Y is not due to lanthanoid contraction.

2. Zr and Nb have similar oxidation state:

Lanthanoid contraction does not directly influence the oxidation states of elements. Zirconium (Zr) and Niobium (Nb) are adjacent elements in the periodic table, and while they may exhibit some similar oxidation states, this similarity is not primarily due to lanthanoid contraction.

3. Zr and Hf have almost the same radius:

This statement is correct. Zirconium (Zr) and Hafnium (Hf) are located in the same group (Group 4) of the periodic table, and they exhibit almost identical atomic and ionic radii due to lanthanoid contraction. The contraction in size caused by the poor shielding of the 4f electrons results in Zr and Hf having very similar radii despite being in different periods.

4. Zr and Zn have the same oxidation state:

This statement is incorrect. Zirconium (Zr) and Zinc (Zn) are different elements with distinct properties and oxidation states. The similarity in oxidation states between these two elements is not attributed to lanthanoid contraction.

The correct statement is 3. Zr and Hf have almost the same radius.

This statement accurately reflects the impact of lanthanoid contraction on the radii of Zirconium (Zr) and Hafnium (Hf), resulting in them having almost identical atomic and ionic radii despite being in different periods of the periodic table.