Sally's apartment lease has expired, but she can stay until the owner's parents move in. The type of tenancy is:

The correct answer is the tenancy at will. This type of tenancy exists when a tenant occupies a property with the consent of the landlord but without a formal lease agreement. In this scenario, Sally’s lease has expired, and she is allowed to remain in the apartment until the owner's parents move in. This arrangement implies a level of agreement between Sally and the landlord, where the landlord can terminate her tenancy at any time.

Tenancies at will are typically flexible, allowing tenants to stay on the property without a defined end date, as long as both parties are in agreement. It is important to note that while Sally has the right to reside in the property temporarily, that right is contingent on the landlord's discretion, making this type of tenancy more informal than others, such as an estate for years or a periodic estate, which have defined terms.

A periodic estate would involve a lease that automatically renews for successive periods, while an estate at sufferance occurs when a tenant remains in possession of the property after the lease has expired without the landlord's consent. Fee simple refers to a form of ownership rather than a tenancy. This context clarifies why tenancy at will is the most fitting description of Sally's situation.