What is the definition of an observation unit in emergency services?

An observation unit is a separate unit within a hospital that provides care for patients who require more than 24-48 hours of observation.
An observation unit is a virtual unit that can be located anywhere in the hospital.
An observation unit is a unit within or near the emergency service that has between 10 and 20 beds for patients who need more than 4-8 hours of observation.
An observation unit is a unit within the emergency service that is staffed by a variety of specialists and teams.
a) I and II are correct.
b) II and III are correct.
c) III only is correct.
d) II only is correct.
e) III and IV are correct.