To answer the question, let us observe the following from the map.
Red stars indicate grocery stores that opened in 2007.
Blue circles indicate stores that opened in 2008.
Red stars = 8 grocery store in 2007
Blue circles = 24 stores in 2008
Therefore, from the above observation, the number of stores in the city had increased from 8 to 32, between 2007 to 2008.
It is believed that an increase in population brings about economic development to any community, in other words, Population growth must have caused a need for more stores in the city. Or why would anyone want to open a store in a disserted city?.
Therefore, the answer is D. The population of the area has likely increased.