What does the term millennium refer to? a. a period of 100 years b. a period of 1,000 years c. any long period of time d. an important period of time

B. A period of 1,000 years. this is the correct answer.

A millennium is a period of 1,000 years, for example the year 2000 was the new millennium.
 
An era is an important period of time, where as a century is a period of 100 years, there is no name for "any long period of time", so none of these are the correct choices.