The answer is B. Year.
A calendar is a system of organizing days for social, religious, commercial, or administrative purposes. A date is a specific point in time, conventionally expressed as the number of days since the beginning of a particular calendar system. A month is a unit of time for a calendar, having a definite number of days and months in a year. A day is the basic unit of time in the International System of Units (SI) and is usually 24 hours long.
A year is a period of 365 or 366 days, a solar year being the time it takes the Earth to orbit the Sun. A year is a common unit of time in the Gregorian calendar, which is the most widely used calendar in the world.
The other four options (A, C, D, and E) are all units of time that are shorter than a year.