Which word best describes a decrease in size or extent?

The word that best describes a decrease in size or extent is "diminish." This term specifically means to make or become less, which directly relates to a reduction in size, quantity, or importance. When something diminishes, it is undergoing a process of becoming smaller or less significant.

In contrast, the other choices suggest an increase or an expansion. "Augment" refers to the act of making something greater or larger, while "increase" denotes a rise in size, amount, or degree. "Expand" indicates the action of increasing in size, volume, or scope. Each of these terms conveys a sense of growth or enlargement, which is the opposite of a decrease. Thus, "diminish" is the most appropriate choice for describing a reduction.