Hello,
The author says "There will be a noticeable negative economic effect on these businesses"
This he says because the percentage who retired to Florida has decreased by three percentage points over the past ten years
Let's understand what this means - Out of all the retirees who retired to different locations the percentage of people who retired to florida has dropped by 3%.
For instance if the percent of retirees retiring to florida was 33% it's probably dropped to 30% now. The assumption here is that the total number of retirees now and 10 years back is more or less the same.
Lets consider that 10 years back there were 10,000 total retirees; now there are probably 20,000. Is 33% or 10,000 bigger than 30% of 20,000? nope!
Option D points out exactly this. Thus option D is Correct.
Hope this helps!