Final answer:
The correct answer is B. Unstable.
Step-by-step explanation:
In meteorology, the stability of the atmosphere refers to its tendency to resist vertical motion. Cumulus clouds that continue to develop to great altitudes indicate an unstable atmosphere. This is because the rising air within the cumulus clouds promotes further cloud development.
Unstable atmosphere allows the air parcels to rise freely, leading to the formation of towering cumulus clouds and potentially unstable weather conditions like thunderstorms.
Therefore, the correct answer is B. Unstable.