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(c ) Covalent radius.

Explanation

Atomic radius refer to both metallic radius and covalent radius.
(c ) Covalent radius: It is defined as one half of the equilibrium distance between the nuclei of two atoms bonded by a covalent bond.
Ex. Covalent radius of hydrogen = 0.37 Å
Covalent radius of chlorine = 0.99 Å

AB = covalent radius = 0.99 Å
CD = van der Waal's radius = 1.8 Å
Covalent,van der Waal's radii of cholrine.
van der Waal's radius: It is defined as one-half of the inter-nuclear distance between atoms of two adjacent molecules of an element bonded by van der Waal's forces facing each other, in solid state.
Ex. van der Waal's radius of hydrogen = 1.2 Å
van der Waal's radius of chlorine = 1.80 Å
Note: van der Waal's radius of an atom is 40% larger than its covalent radius.