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 partially filled d-orbitals and exhibit certain characteristic properties such as variety of oxidation states, formation of coloured ions, act as catalyst and slow paramagnetic behavior.

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 in any series.

The catalytic activity of transition metals and their compounds is ascribed mainly to

their ability to adopt variable oxidation states.

The correct answer is (1) their ability to adopt variable oxidation states.

The catalytic activity of transition metals and their compounds is ascribed mainly to their ability to adopt variable oxidation states. This means that they can easily change the number of electrons they lose or gain when forming bonds with other atoms. This makes them very good at activating reactant molecules and lowering the activation energy of reactions.

The other options are incorrect:

(2) is incorrect because while transition metals are generally chemically reactive, this is not the main reason for their catalytic activity.

(3) is incorrect because the magnetic behavior of transition metals is not directly related to their catalytic activity.

(4) is incorrect because the outermost orbital of transition metals can have more than two electrons.

Here are some examples of transition metals and their compounds that are used as catalysts:

• Iron is used as a catalyst in the Haber process, which is used to produce ammonia fertilizer.
• Nickel is used as a catalyst in the hydrogenation of vegetable oils, which is used to produce margarine and other processed foods.
• Platinum is used as a catalyst in the catalytic converter, which is used to reduce emissions from cars.

Transition metals and their compounds are essential for many important industrial processes, and their catalytic activity is one of their most important properties.