Choose the word which is different from the rest.

The answer is C. Calendar.

A day is a unit of time equal to 24 hours, or one rotation of the Earth on its axis. A fortnight is a period of two weeks, or 14 days. A month is a period of time equal to one cycle of the Moon’s orbit around the Earth, or 28, 29, 30, or 31 days. A calendar is a system of organizing days for social, religious, commercial, or administrative purposes.

A calendar is different from the other options because it is not a unit of time. It is a system of organizing days.