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 degrees of longitude per hour.
D) Earth does not rotate; rather, it revolves.