Answer:
c. the Prime Meridian
Step-by-step explanation:
The zero longitude is called the Prime Meridian, also known and often referred to as Greenwich. At the Prime Meridian, the longitude is zero because it is the imaginary line on the coordinate systems where the longitude starts from. Anything on this particular line has 0 degrees longitude, while everything east and west from it has certain number of degrees in longitude, the closer it is to the Prime Meridian, the smaller the degrees are, the further away the place is, the more degrees of longitude there are.