Final answer:
National air carriers typically have variable annual revenues due to the fluctuating nature of factors affecting the airline industry, despite potentially high revenues owing to their large scale of operations. Option c.
Step-by-step explanation:
The question pertains to the characterization of national air carriers' annual revenues. While the revenue can be affected by numerous factors such as economic conditions, competition, and fuel prices, it is generally not appropriate to describe it as inherently high, low, constant, or variable without context.
However, one might argue that in comparison to smaller carriers or other types of businesses, national air carriers could have high annual revenues due to the scale and breadth of their operations.
Yet, these revenues are subject to fluctuate or be variable due to the dynamic nature of the airline industry, which is impacted by factors such as seasonal travel demand, economic cycles, and geopolitical events.
As a result, the most accurate choice for a national air carrier's annual revenues would generally be variable, reflecting the changes and uncertainties in the airline market. So option c.