What imaginary line is used to measure degrees east and west of the prime meridian?

The prime meridian is the reference point for measuring longitude, which determines degrees east and west. Meridians are the imaginary vertical lines that run from the North Pole to the South Pole and represent these degrees of longitude. They are used to indicate the exact position of a place in relation to the prime meridian.

The equator, on the other hand, is the primary reference line for measuring latitude, which is concerned with degrees north and south of the equator. Parallels are lines of latitude that run parallel to the equator but do not measure east or west. Latitudes also refer to this concept of measurement in reference to the equator.

Thus, the correct answer identifies meridians as the lines that relate directly to measuring east and west of the prime meridian, making it the appropriate choice.