What does it mean to "sap"?

The word "sap" typically refers to the act of depleting or draining resources, especially in the context of energy, strength, or vitality. When something is sapped, it loses its power or effectiveness due to being diminished. This can apply to physical resources, such as energy or materials, as well as intangible resources like morale or enthusiasm.

In contrast, the other choices involve processes that build or enhance something rather than drain or diminish it. For instance, building up or enriching suggests increasing quantity or quality, and increasing vitality implies enhancing life force or energy. None of these actions align with the essence of "sap," which is inherently related to the reduction of available resources. Thus, the choice that aligns most closely with the definition of "sap" is indeed the option that describes the act of depleting resources.