According to the dichotomous key, stentor would be a member of the kingdom

The answer is Protista.

The dichotomous key is used to identify the taxonomic category of an organism. So, if we start with statements from the beginning, and discard the possibilities, we can put the organism into some taxonomic group.

Accordingly, we know that a stentor has a nucleus, so it must be eukaryotic. Among the Eukaryotes, it could belong to the kingdom of Protista, Fungi, Plantae and Animalia. Since it is unicellular organism and lives only in the water, it could belong only to the kingdom of Protista