What imaginary line is located at 0° longitude?

equator
north pole
prime meridian
south pole

The answer is: [C]: the Prime Meridian.

The Prime Meridian; which is an imaginary line and arbitrarily chosen, runs through 0° longitude and runs through Greenwich, English.

The Equator runs through 0° latitude.

The North Pole is located at 90° north latitude and is not an imaginary line.
Technically, any longitude value from 0° to 180°—East or West—would be the correct longitude for the North Pole; however, by convention, the coordinates are 90°N, 0°W.

The South Pole is located at 90° south latitude and is not an imaginary line. Technically, like the North Pole, any longitude value from 0° to 180°—East or West—would be the correct longitude for the South Pole; however, by convention, the coordinates of 90°S, 0°E; or even: - 90° S, 0° E.