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(b) Metallic radius.

Explanation

Covalent radius: It is used generally for non-metals. Covalent radius is half the equilibrium distance between the nuclei of two atoms with a covalent bond. Covalent radii of two atoms can be added to give internuclear distance between them. E.g: Covalent radius of H is `0.37Å` and for chlorine it is `0.99Å`. Hence internuclear distance between H and Cl in HCl is `1.36Å`.