Today one of Mexico City's greatest technology challenges is

Sinking land

Explanation:

Mexico City has sunk around 10 meters over the last 100 years. Structures such as buildings and churches are leaning, and the land has made an uneven descent into the earth. Mexico City sinks about three feet a year now.

Mexico City, Mexico has numerous challenges in technology, crime, jobs, etc. Their greatest technological challenge today is sinking land. In the last century alone, Mexico City has shrunk 30 feet. The correct answer is B.