The kings of the sumerian city-states acted as intermediaries between whom?

The correct answer is - between the people and the Anunnaki.

The kings of the Sumerians were considered as half-Gods, as a mixture between a human and the Anunnaki. The Anunnaki were considered as Gods, as to whom it was attributed everything they new, being architecture, math, astrology, written letter, way of life... The Anunnaki are represented as human-like in appearance, but much bigger. Initially they were ruling over the people, but than decided to create a human-Anunnaki hybrid, so that it can be smarter and wiser than the rest of the humans, so that the king can rule easily and properly.

d. migration