Which government type is best described as being ruled by a small group, such as the wealthy or the military?

The correct answer is D. Oligarchy.

An oligarchy is a form of government where a small group of people, often distinguished by wealth, military power, or other elites, hold power and make decisions for the larger population. This contrasts with other forms of government like monarchy, where a single ruler holds power, or democracy, where power is derived from the people.