The Basis for Defining the Length of a Day Is

The basis for defining the length of a day is the fact that

A) Earth rotates east to west.
B) Earth moves through 365.25 days a year in its orbit about the Sun.
C) Earth rotates on its axis in 24 hours i.e. ,it rotates 15° of longitude per hour.
D) Earth does not rotate rather,it revolves.
E) Earth rotates west to east.