The hardness of Transition elements is due to:

Metallic bonding

The hardness of transition elements is primarily due to (2) metallic bonding.

Transition elements are characterized by the presence of partially filled d orbitals, which allow for the formation of metallic bonds. Metallic bonding occurs when the valence electrons of metal atoms are delocalized and shared among all the metal atoms in a crystal lattice.

The delocalized electrons in metallic bonding contribute to the properties of metals, including their hardness. These electrons are free to move throughout the metallic lattice, creating a "sea" of electrons that can absorb and transmit energy. This contributes to the strength and rigidity of the metal, resulting in its hardness.

On the other hand, factors such as large atomic size (1), covalent bonding (3), and high ionization energy (4) do not directly contribute to the hardness of transition elements. Large atomic size may affect other properties like density, but it does not directly influence hardness. Covalent bonding is typically observed in nonmetals rather than transition metals. High ionization energy refers to the energy required to remove an electron from an atom, which is not directly related to the hardness of the metal.

Therefore, the primary reason for the hardness of transition elements is (2) metallic bonding.