First thing to note is that the answer MUST be true given the facts.

A) We are told that over 95 percent of people who have asthma suffer from hay fever, so it could be 96 percent which shows that hay fever is not a prerequisite for development of asthma, as for this to even be considered the case it should be 100 percent.

B) This is similar to B, but also knowing that asthma is less common than hay fever rules this out.

C)We have no way of proving this from the data given.

D)The number of people with asthma is less common than the number of people with hay fever and 95% of these have hay fever so there is no way of proving whether this is true. As we do not know the percent of people with the more common hay fever who have asthma.

E) This is true as hay fever is much more common (read as there being a LOT more hay fever sufferers) so even if 100 percent of people who had asthma had hay fever, this would not be 95% of hay fever sufferers. Because then the statement would read that they are almost as common.

Simplified (using numbers):
Hay FeverMUCH more common than Asthma (1000 hay fever vs 100 asthma)
95% of Asthma sufferers have hay fever (95 have both)
So less than 95% who have the more common hay fever have asthma (1000 hay fever, 95 both)