Which party would use the remedy of "unlawful detainer"?

The remedy of "unlawful detainer" is typically used by a lessor, which refers to the landlord or property owner. This legal process is initiated when a tenant (lessee) remains in possession of a property after their lease has expired or after they have failed to comply with the terms of the lease, such as not paying rent. The lessor files for unlawful detainer to seek formal eviction of the tenant from the property.

Using this remedy allows the lessor to regain possession of their property legally and ensures that the tenant is removed in accordance with local laws. Given that the remedy is aimed at addressing situations where the tenant refuses to leave despite the lease terms, it is inherently the lessor who would take this legal action. Thus, the correct answer reflects the party that has the legal standing to pursue this remedy.