In the given case, the most appropriate step to be taken by Brian in order to formulate the hypothesis is to visit other households with cats. In the given case, Brian has to confirm that whether he is allergic to cats or not, and for this, he must go on to find if a similar sneezing reaction is experienced when he is around other cats apart from aunt's cat.
It can be a case that some other environmental elements may have been resulting in the condition. Thus, wearing a mask would not help the cause straightforwardly as it would not confirm that Brian is allergic to cats.
