Which of the following requires a minimum equipment list (MEL) to be valid?

A minimum equipment list (MEL) is a document that specifies the appliances and equipment that must be operative for an aircraft to be considered airworthy within specific operational constraints. For aircraft operating under a standard airworthiness certificate, the MEL is critical because it provides guidelines for determining which equipment failures can be tolerated without compromising safety.

This is particularly important as it ensures that the operator can maintain compliance with safety regulations while managing operational needs. The standard airworthiness certificate signifies that the aircraft meets the required standards set by aviation authorities, and therefore, should have an MEL to ensure that any inoperative equipment does not negatively affect the aircraft's safety or performance.

In contrast, other scenarios mentioned—such as operating under special authorization, flying international routes only, or operating in reduced visibility conditions—do not inherently require the establishment of an MEL. These situations may have specific operational requirements, but they do not mandate compliance with an MEL in the same way that the standard airworthiness certificate does.