Question

vertical lines that measure east to west of the prime meridian
equator
latitude
longitude
prime meridian

Explanation:

Longitude lines are vertical (north - south running) and measure east - west positions relative to the Prime Meridian. The Equator is a horizontal line dividing the Earth into northern and southern hemispheres. Latitude lines are horizontal and measure north - south positions. The Prime Meridian is the reference line for longitude, not the lines that measure east - west from it.

Answer:

C. Longitude