Answer: C. Greeks looked for places with resources to start new colonies.
The first option is not related to the issue of overpopulation; it only deals with concerns regarding food resources and cheap labor.
The second option similarly does not have any relation to concerns about overpopulation.
The last option deals with problems of land, which could potentially be related to overpopulation, but the statement does not imply it.
Option C is the only one that deals with overpopulation, as trying to find new land for settlements could be a consequence of shortage of land because of a high population.