I've seen this question before on this forum, and I've always thought it was a bit of a ridiculous question. We have information about estimates of income and population, but are the estimates accurate? We have no idea. I can't imagine seeing a real GMAT question worded in this way- instead, it would more likely say:
1) The population is greater than...
2) The total income is greater than...
Still, the answer is E regardless of the wording- you would need the upper limit on the population and the lower limit on total income to get the minimum possible per capita income (per capita income = total income/population).
Anyway, I'm curious about the source of the question.