Answer is: D. High ionization energies; high electron affinities.
Covalent bond is usually between nonmetals.
The ionization energy (Ei) is the minimum amount of energy required to remove the valence electron, when element lose electrons, oxidation number of element grows (oxidation process).
Nonmetals are far right in the main group and they have highest ionization energy, because they have many valence electrons.
For example, fluorine has the greatest affinity for electrons and tendency to be reduced, than oxygen and chlorine.
Fluorine form molecule of fluorine F₂ with single nonpolar covalent bond.