Which word means to reduce in amount or degree?

The correct choice is "Abate," which means to lessen or diminish in amount, degree, or intensity. This term is often used in contexts where something that was previously strong or severe is being reduced, such as the abatement of a storm or noise.

While "decrease" might also convey a similar notion of reduction, "abate" is more specific to a lessening of something that is often viewed as negative or excessive. "Modify" refers to making changes or alterations, which doesn’t inherently imply a reduction. "Mitigate" means to make something less severe or serious but does not directly convey a reduction in amount or degree; instead, it often relates to softening the impact of a negative situation. Thus, "Abate" is the precise term that encapsulates the idea of reduction.