The most likely pair to form a principally covalent bond is sulfur and oxygen. This is because both sulfur and oxygen are nonmetals, and when two nonmetals bond, they tend to share electrons more equally, resulting in a covalent bond.
In contrast, calcium and rubidium are metals, and when they bond with nonmetals like oxygen and chlorine respectively, they typically form ionic bonds. Ionic bonds occur when there is a significant difference in electronegativity between the elements involved, leading to the transfer of electrons from the metal to the nonmetal.
On the other hand, lithium and chlorine, while also creating an ionic bond due to lithium being a metal and chlorine being a nonmetal, will not exhibit covalent character to the same extent as the bond between sulfur and oxygen. Therefore, the bond between sulfur and oxygen is the best example of a principally covalent bond among the given options.