Which of the following describes the extreme right or left of an element?

The term that best describes the extreme right or left of an element is "flank." In a military context, the flank refers to the sides of a formation or unit. Each side of a unit is considered its flank, and maneuvering operations often involve protecting these flanks from enemy attacks or exploiting them for strategic advantage. Understanding the concept of flanking is crucial, as it helps military leaders direct their forces effectively and maintain formations during combat.

The other terms, while they may have some relevance in different contexts, do not specifically denote the extreme sides of an element in a military formation. "End" generally refers to the termination point of an object or formation, "edge" tends to be used in a more general sense to describe the outer limit of anything, and "boundary" typically refers to an invisible line that marks the limits of an area. None of these terms carry the specific tactical connotation that "flank" does in military operations.