The stem states that over 95% of people with asthma also have hay fever. This implies that nearly all asthma sufferers are part of the larger group of hay fever sufferers. If asthma were as common or more common than hay fever, the percentage of hay fever sufferers who also have asthma would be close to or above 95%. However, because asthma is much less common than hay fever, the set of hay fever sufferers is much larger, and therefore, the percentage of hay fever sufferers who also have asthma is much lower. Hence, answer (B) correctly explains the observed percentages.
Let's test some numbers
Asthma: 100
Asthma patients who also have HF: 96 (over 95% of all Asthama patients)
B says, the total no. of people who have HF is "MUCH higher" than the total no. of people who have asthma. Let's use extreme values to test this
Total Hay Fever patients: 1000
Total Asthama patients: 100
% = 10% (which is lower than 95%)
Now, technically we can say use these numbers, even though a 10% diff is not necessarily much higher, but note that as 95% of 900 people would need to have HF as per the statement in the stem, as the no. of Asthama patients go up, so will the no. of hay fever patients, but the opposite is not necessarily the case.
Total Hay Fever patients: 1000
Total Asthama patients: 900
% = 90% (which is lower than 95%)