If the incentive does not grow enough to cause the Moon colonies to be built, then that shows we cannot conclude the colonies will be built simply based on the incentive. So, the author does have to assume what (A) describes in order for the premise to support the conclusion.

The author does not assume there are no other ways to relieve overcrowding on Earth. The author’s position is simply that Moon colonies will be among the methods used to relieve overcrowding, not that other methods won’t be tried.

The conclusion is that the colonies will be built, so agreeing that they’ll be built doesn’t hurt the argument. (C) could be correct if the conclusion were that the colonies will be built because of the incentive. But, the incentive part is in the premise, not the conclusion.

This possibility doen’t undermine the argument, becaue the conclusion only concerns overcrowding on Earth. Even if the Moon becomes overcrowded, there would still be at least some relief from overcrowding on Earth due to the Moon colonies.

The argument doesn’t assume anything about people’s preferences. There might be some people who prefer to live on the Moon right now, even when Earth isn’t overcrowded. That wouldn’t impact the relationship between an economic incentive and the building of the colonies.