Official Explanation
The hand-held vacuum cleaner would only clear small areas. Homeowners will need to own conventional vacuum cleaners. We need to explain --- if folks own a conventional vacuum cleaner, why would they also need this hand-held cleaner?
(C) is the credited response. If there are places that a convention vacuum cleaner could not reach and clean, but which the hand-hold cleaner could reach and clean, then that provides a cogent reason for owning the hand-hold cleaner in addition to a conventional vacuum cleaner.
(A) is a weakener, and we are asked to strengthen the argument. If some types of dirt do not need tremendous power, then this in and of itself provides no incentive to buy the powerful hand-held cleaner.
(B) is irrelevant. An older conventional vacuum cleaner can only be replaced with a new conventional vacuum cleaner. We are told explicitly: the hand-held cleaners will not take the place of a conventional vacuum cleaner. Therefore, this transaction --- replacing an old vacuum cleaner with a new one --- does not involve the hand-held cleaners at all.
(D) is irrelevant. The hand-held cleaner is only used for small areas --- it isn't even used for large areas.
(E) is irrelevant. It doesn't matter who cleans the house. This argument is about whether a certain cleaning product would be needed.