A sentinel event refers to which situation?

A sentinel event refers to which situation?
a. An event that could have harmed a patient, but serious harm didn't occur because of chance.
b. An event that harms a patient as a result of underlying disease or condition.
c. An event that harms a patient by omission or commission, not an underlying disease or condition.
d. An event that signals the need for immediate investigation and response.
Answer D

situation?