Answer the question on the basis of passage given below:
In the periodic table, the d-block contains the elements of group \(3\) to \(12\). The d-orbitals are progressively filled in each of the four long periods. The elements of d block referred as transition metals have partly filled d orbitals and exhibit certain characteristic properties such as variety of oxidation states, formation of coloured ions, act as catalyst and show paramagnetic behaviour.
The two inner transition metals series \(4f\) and \(5f\) are known as Lanthanoids and Actinoids respectively. The lanthanoids resemble one another more closely as compared to ordinary transition elements in any series.
The catalytic activity of transition metals and their compounds is ascribed mainly to:

1. A Their ability to adopt variable oxidation states.
2. B Their chemical reactivity.
3. C Their magnetic behaviour.
4. D Their outer most orbital which has two electrons.