The correct answers are:
A. Overcrowding;
The fact that millions of people live in a same urban area, with thousands of people living in a single square mile, resulted in overcrowding. The large urban places especially are extremely crowded, with the traffic being terrible because of it, the stores, sidewalks, institutions all being full with people all the time.
C. Pollution;
The more people there are, the bigger pollution, it is like an unwritten rule. Also, the urban places are heavily industrialized. The combination of those two creates a lot of pollution.
D. Loss of farmland;
Most of the urban places have been built, and continue to spread out on fertile land. That has led to a significant loss of fertile farmlands, thus creating a problem in the food production.