Which in-flight advisory would contain information on severe icing not associated with thunderstorms?

The choice related to severe icing not associated with thunderstorms is indeed a SIGMET. SIGMETs are significant meteorological information advisories that inform pilots about hazardous weather conditions affecting safety in flight, such as severe turbulence, severe icing, and volcanic ash.

Specifically, a SIGMET for icing will provide information on widespread areas of severe icing that could pose a risk to aircraft operations, excluding any conditions associated with thunderstorms. This is crucial for pilots to adjust their flight plans accordingly to avoid dangerous weather.

Other types of advisories mentioned, while they serve specific purposes in flight planning and safety, do not focus on the same level of severe conditions or the same scope of areas as SIGMETs. For instance, Convective SIGMETs are intended to provide information about thunderstorms and associated severe weather, not for general icing conditions. AIRMETs are typically used for less severe weather conditions and are designed to inform pilots of significant weather that could affect flight but that is not as severe as the phenomena covered by SIGMETs. Aerodrome Weather Reports are localized reports that provide current weather conditions at an airport but do not convey information on in-flight advisories for severe weather phenomena.

Thus, the SIGMET is the most appropriate advisory for significant in