What are the d-block elements called? 

Transition elements

The correct answer is option 3. Transition elements.

Transition elements, also known as transition metals, are elements found in the d-block of the periodic table. These elements are characterized by the filling of d orbitals in their electron configurations. The d-block is situated between the s-block (to the left) and the p-block (to the right) on the periodic table.

Key characteristics of transition elements include:

1. Variable Oxidation States: Transition elements commonly exhibit multiple oxidation states. This flexibility in oxidation states is due to the availability of electrons in both the outer s and inner d orbitals.

2. Formation of Colored Compounds: Transition metal compounds often display vibrant colors, which arise from the absorption and emission of light by electrons in the d orbitals.

3. Formation of Complex Ions: Transition metals have a high tendency to form complex ions by coordinating with ligands. These complexes often exhibit unique properties.

4. Paramagnetism: Many transition metals are paramagnetic, meaning they are attracted to a magnetic field due to the presence of unpaired electrons.

5. High Melting and Boiling Points: Transition metals generally have high melting and boiling points, reflecting the strength of metallic bonding.

Examples of transition elements include iron (Fe), copper (Cu), zinc (Zn), and chromium (Cr). These elements play crucial roles in various industrial processes, biological systems, and everyday applications due to their unique properties. The d-block elements contribute significantly to the diversity and complexity of the periodic table.