An unlawful detainer (UD) action is instigated by `____`

A a lessor.

explanation

An unlawful detainer (UD) action is the court procedure to force a tenant to vacate the rental space they do not have the right to possess. A lessor (landlord) is the aggrieved person who initiates this process against an individual who holds no right to occupy a property, such as a holdover tenant.