How many years are there in a millennium?

A millennium is defined as a period of one thousand years. The term is derived from the Latin words "mille," which means thousand, and "annus," which means year. Thus, when counting the length of a millennium, it consists of 1,000 continuous years.

In the context of the options provided, this understanding directly aligns with the correct answer. The other figures, such as 100, 500, and 1500 years, do not conform to this definition. A millennium specifically spans the timeframe from the start of one year until the end of the 1000th year, marking a significant milestone in the Gregorian calendar and historical context.