Answer:
c. A coastal area with fertile soil
Explanation:
People are more likely to densely populated a coastal area with fertile soil. On the one hand, coastal area provides access to the water. If it's the coastal area of the sea or ocean it is likely to mean possibility of transportation and goods import / export. The fertile soil, on the other hand means possibility to grow crops, vegetables and fruits that would feed the population. Take the example of California, which is the most populous state in the United States.
