You have asked why (E) isn't the best option. In the argument we find that "... there was no reduction in the number of people suntanning on the beach each day." So although (E) suggests that the percentage of people outdoors reduced, that does not mean that the absolute number of people did so.

Many GMAT questions work by getting people confused about the difference between the percentage and the absolute number. The absolute number can increase while the percentage goes down. Similarly, the percentage can stay constant while the absolute number decreases.

Here's another problem that tends to exploit this oversight in people:

In the United States, of the people who moved from one state to another when they retired, the percentage who retired to Florida has decreased by three percentage points over the past ten years. Since many local businesses in Florida cater to retirees, these declines are likely to have a noticeably negative economic effect on these businesses and therefore on the economy of Florida.

Which of the following, if true, most seriously weakens the argument given?

(A) People who moved from one state to another when they retired moved a greater distance, on average, last year than such people did ten years ago.
(B) People were more likely to retire to North Carolina from another state last year than people were ten years ago.
(C) The number of people who moved from one state to another when they retired has increased significantly over the past ten years.
(D) The number of people who left Florida when they retired to live in another state was greater last year than it was ten years ago.
(E) Florida attracts more people who move from one state to another when they retire than does any other state.

The argument states that the percentage has decreased by 3 percent. The argument then assumes that the total number of retirees must have gone down and concludes that Florida businesses that cater to retirees will be adversely affected.

Answer choice (C) is the one that attacks the assumption that a percentage decrease must have led to a decrease in the total number of retirees moving to Florida.