From the given options, select the one that will rightly substitute the given expression:

‘Period of 10 years’

Decade

Out of the provided options, the one that rightly substitutes "Period of 10 years" is: Decade

Here's why:

• Decade: This word specifically refers to a period of ten years, making it the most direct and accurate match for the given expression.
• Fortnight: This signifies a period of two weeks.
• Annuity: This term refers to a series of regular payments made annually, not a specific duration of time.
• Millennium: This denotes a period of one thousand years.