Covalent bonds usually occur between atoms that have
and
a high ionization energies, high electron affinities
b. high molecular weights large atomic radii
c. low molecular weights, small atomic radi
d. low ionization energies, low electron affinities

A: high ionization energies; high electron affinitlies.

Explanation: Covalent bonds are basically about sharing of electrons between two atoms to achieve that stable structure. They are formed between two atoms when both have similar tendencies to attract electrons to themselves (i.e., when both atoms have identical or fairly similar ionization energies and electron affinities). Covalent bonding usually occurs between two non-metals.

For effective and proper bonding, the two atoms involved in the covalent bonding exercise should be small and hungry for electrons. This is to enable the nuclei of both atoms to effectively attract and hold the shared electron(s) in place; hence, the need for high ionization energies & high electron affinities for a more effective covalent bonding.