Question

Which of these can vary in length of time?

century
decade
millennium
era

Answer 1

An era can vary in length of time because it can refer to a short span a of time, such as a monarch's reign or a long span of time, such as the Mesozoic Era.

Answer 2

That's right^, a century is 100 years. A decade is 10 years and a millennium is 1000 years.