Which types of incidents could trigger a TFR?

A Temporary Flight Restriction (TFR) can be established for a variety of significant incidents or situations that could pose a threat to aviation safety. Each of the scenarios listed in the question can indeed lead to the issuance of a TFR.

Volcanic eruptions lead to the potential for ash clouds that can be dangerous for aircraft engines and airframes, prompting restrictions in the airspace surrounding the eruption to ensure safe operations.

Aircraft incident or accident sites necessitate TFRs to keep the area clear of other traffic, allowing emergency responders to operate safely and preventing any further complications arising from the presence of other aircraft.

Toxic gas leaks or spills represent another scenario that could lead to a TFR as they might pose significant risks not only to the environment but also to the safety of aerial operations in the vicinity.

Given that all these incidents can independently justify the creation of a TFR to protect aviation safety and public welfare, the correct answer encompasses all options provided.