Which characteristic is true about a flare star?

Flare stars are indeed known for their unpredictable brightness changes. This characteristic is primarily due to their magnetic activity, which can lead to sudden and significant increases in brightness, often by several magnitudes. These flares can happen over a few minutes to hours, making them intriguing objects for astronomers studying stellar behavior.

In comparison, the other characteristics do not apply to flare stars. For instance, they are not consistently stable because their brightness can vary dramatically. Likewise, while flare stars may exhibit a range of colors depending on their temperature and composition, they are not always the same color. Additionally, flare stars, especially those that are cool, can have rapid fuel consumption associated with their magnetic activity, which contradicts the idea of burning fuel slowly. Therefore, the unpredictability of their brightness is what sets flare stars apart in stellar classifications.