What is the characteristic of alkali metals?

The characteristic of alkali metals that makes option C the correct choice is their electronic configuration. Alkali metals, found in Group 1 of the periodic table, have one valence electron in their outermost shell. This single electron is responsible for their high reactivity, as they tend to lose it easily to form positive ions (cations) with a charge of +1. This unique electron structure contributes significantly to their chemical behavior and reactivity with other elements.

The other options describe properties that do not apply to alkali metals. For instance, while they do form basic oxides (which relates to option B), that characteristic does not define them as uniquely as their single valence electron. Inertness and non-reactivity (option A) contradict the fundamental nature of alkali metals, making them among the most reactive metals. Additionally, describing alkali metals as poor conductors of electricity (option D) is inaccurate, as they are known to be good conductors due to the mobility of their valence electrons.