Which factor is the primary reason urbanization occurs?

Population growth is the primary reason for urbanization to occurs.

Explanation:

When there is an increase in the population of an area, there is a subsequent decrease in the availability of regional resources. Owing to the scarcity of resources, populations migrate to the urban areas in search of opportunities to fetch more resources to themselves. This eventually leads to greater degree of urbanization of the already urban area. Urban areas expand to make room for more population resulting in more urbanization.

Well, a and b together would be negative. b alone would also be negative.