All the options listed can be considered significant issues in Mexico City, but if you're looking for an example specifically related to infrastructure problems, "insufficient transportation systems" would be the most direct answer. This reflects a challenge in the infrastructure required to support the city's large population and reduce congestion and pollution.
However, both air pollution from vehicle emissions and urban overcrowding can also be indirectly related to infrastructure issues, as they may result from inadequate transportation planning and urban development. Income inequality is more of a social issue rather than an infrastructure problem.
So, to directly answer your question: insufficient transportation systems.