Which weather advisory is meant to inform about severe conditions affecting tree-level flying?

The correct choice is SIGMET. SIGMETs are issued primarily to inform pilots of severe weather conditions that may affect the safety of flight, including phenomena such as turbulence, icing, thunderstorms, and volcanic ash. These advisories are particularly critical for pilots operating at tree-top level or in surroundings that may not be suitable for safe flying due to the significant impact of these conditions.

In contrast, AIRMETs are concerned with less severe weather, providing information about conditions such as light turbulence or icing that, while still pertinent, do not typically warrant the urgency of a SIGMET. Center Weather Advisories are used to alert pilots to significant weather changes based on real-time observations, but are less comprehensive than SIGMETs. Convective SIGMETs specifically address severe convective weather phenomena, such as thunderstorms, but do not cover broader weather hazards that could also endanger low-level flights.