What must be done to handle stablizer trim runaway effectively?

Handling stabilizer trim runaway requires immediate and effective action to regain control of the aircraft. Changing the flight mode to manual control is critical in this scenario because it allows the pilot to override the automated systems that are causing the runaway situation. By switching to manual control, the pilot can take direct command of the aircraft's pitch and stabilizer settings, restoring the ability to maneuver the aircraft safely.

In contrast, the other options may not effectively address the trim runaway condition. Automatically adjusting trim settings can exacerbate the problem if the underlying issue persists. Lowering altitude immediately may not resolve the runaway condition and could create other risks, such as increased stall potential or running out of airspace for maneuvering. Performing a go-around, while useful in other situations, isn’t a direct response to trim runaway and may not ensure the aircraft's stability.

Thus, manually taking control is the best immediate action to manage stabilizer trim runaway effectively, enabling the pilot to assess the situation and respond appropriately.