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(c ) Covalent radius.

Explanation

Vander Waals radius (Collision radius): The Vander Waals radius is half the equilibrium distance between the nuclei of two atoms bound by Vander Waals forces. It is used for molecular substances in solid state and for inert gases. E.g: The vander Waals radius for hydrogen is `1.2Å` and that of chlorine is `1.80Å`. The Vander Waals radius of an atom is 40% larger than its covalent radius.