So "expanded tax base" is referring to the increase in total taxable income that will occur when richer families move into the neighborhood (gentrification) and this is specifically an incentive to local government. The reason affordable housing isn't going to be developed is because the land value goes up, so developers come in and just make more expensive, lucrative developments that aren't going to be affordable, but will encourage the influx of more wealthy people. That's why A is the answer, gentrification, by definition, makes an affordable/low-income area much more expensive, which can force out the residents. Any other confusion on this one?