Hermione thinks she has discovered a protection spell that will reduce the pain of the Cruciatus curse. She believes that a sample of people protected by her spell will rate their pain as less severe than the average person who does not have her protection spell. Her null hypothesis for this study would

There will be no difference in pain ratings from people protected by her spell and people who aren't protected by her spell.

Step-by-step explanation:

The claim in this scenario is the alternative hypothesis, which is what we wish to test ; the alternative hypothesis is thus ; people who are protected by spell will rate their pain as a result of cruciatus curse as less severe than the average person who does not use her protection spell ;

As we know, the null hypothesis is the opposite of the alternative hypothesis and the initial truth, which is ;

The average pain felt by both the user of the pain protections spell and those who do not feel the same amount of pain.