Which of the following elements is classified as an actinoid group element?

The correct option is Thorium.

Explanation:

In the modern periodic table, elements with atomic numbers from 89 to 103 are classified as actinoids (or actinides). These elements belong to the 5f-block series.

Let's analyze the given options:

1. Thorium (Th): Has an atomic number of 90, which lies in the range 89–103. Therefore, Thorium is an actinoid group element.

2. Lanthanum (La): Has an atomic number of 57. It is a d-block element, though it is closely associated with the lanthanoid series.

3. Cerium (Ce): Has an atomic number of 58, belonging to the lanthanoid series (4f-block).

4. Ytterbium (Yb): Has an atomic number of 70, which also belongs to the lanthanoid series (4f-block).

Thus, among the choices provided, Thorium is classified as an actinoid group element.