The correct answer is C) Altostratus. Altostratus clouds are a reliable indicator of an approaching weather system, typically a warm or occluded front. These mid-level clouds often signal the arrival of a larger, more organized weather pattern that can bring precipitation, wind, and temperature changes. Altostratus clouds are relatively uniform in appearance, covering the entire sky, and their presence is a strong sign that a more significant change in weather conditions is on the way. The other options are not as reliable for indicating an approaching weather system. Cumulostratus clouds are not a standard cloud type, and cumulus and nimbus clouds are associated with more localized or convective weather patterns, rather than larger frontal systems.