see lets solve it in a reverse order :-
(d) metallic radius is only in case of metals, not in any other. so it is not the answer.
(c) Vander waals radius is only in case of noble gases. so this also not the answer.
(b) For ionic radius there should be the electronegativity difference of at least 2. This is also not possible in case of a metalloid.
(a) the last option is the COVALENT BOND. This is the type of bonding which leads to the formation of COVALENT RADIUS.