Which of the following describes an unplanned start-up?

An unplanned start-up is characterized as a startup occurring in an emergency situation. This definition highlights the unexpected nature and urgency associated with unplanned start-ups, typically arising from unforeseen circumstances that require immediate action to initiate processes or systems.

In contrast, a regular scheduled startup, a delayed startup, and a routine planned startup all involve premeditated and organized procedures. These scenarios typically follow established protocols and are often part of standard operating procedures, allowing for adequate preparations, risk assessments, and personnel readiness. They do not encompass the chaotic and sometimes risky aspects that can accompany an emergency response, which is inherent to the definition of an unplanned start-up.