Which of the following is a characteristic of a strong reducing agent?

A strong reducing agent is characterized by its ability to lose electrons easily. This property is critical in redox (reduction-oxidation) reactions because reducing agents donate electrons to other species, thereby causing those species to be reduced.

When a substance can readily lose electrons, it can facilitate the reduction of another substance while itself being oxidized. This is a fundamental aspect of its role in chemical reactions, particularly in analytical chemistry techniques that involve redox processes.

Other options do not align with the inherent qualities of strong reducing agents. For example, high electron affinity refers to a substance's tendency to gain electrons, which is characteristic of oxidizing agents, not reducing agents. High density does not influence the reactivity as a reducing agent in terms of electron donation.