The correct answer is - a. land to sink.
Mexico City is a very city, it is fact the second largest city in the world. It's sheer size means that all the things built by the people are putting a lot of pressure on the land itself.
Considering the fact that Mexico City is built on a place where until relatively recently there was a lake is a big concern. The soil is very soft for starters, and also there's lot of groundwater reserves very close to the surface. By removing the groundwater, big gaps, empty spaces, inside the ground, and close to the surface, are opening. The soft sediments are not able to sustain the pressure, so the land is sinking.
