The red line running north and south on this map marks the location of
a.
the tropic of cancer
b.
the tropic of capricorn
c.
the equator
d.
the prime meridian

The correct answer is D. The red line running north and south on this map marks the location of the Prime Meridian.

Explanation:

Of all possible meridians, the prime meridian is the one designated as the benchmark and the zero line. This zero line is important for cartography, navigation and surveying, because together with the equator it provides the reference point for the geographic coordinates with which positioning on earth is possible. Where the equator is a geographically obvious choice, since it is at odds with the earth's rotation and is exactly in the middle between the poles, the choice of a certain prime meridian is arbitrary and is determined more politically or practically than geographically.

In 1884, the Greenwich Meridian was chosen as a global zero meridian, although in practice the Irish reference meridian is usually used, which deviates slightly from that.