Final answer:
Freezing air temperature limits net primary productivity in both terrestrial and aquatic ecosystems (option c) by affecting plant metabolism and reducing light penetration in water.
Step-by-step explanation:
Freezing air temperature is a factor that can be most limiting to net primary productivity in both terrestrial and aquatic ecosystems. In terrestrial ecosystems, temperature extremes can limit the primary productivity by directly affecting plant metabolism and the length of the growing season. Extreme cold can lead to a reduced rate of photosynthesis or even cessation of growth. In aquatic ecosystems, freezing temperatures can limit productivity by forming ice cover that reduces light penetration, inhibits gas exchange, and can cause physical damage to aquatic plants. Thus, freezing air temperature is indeed limiting to net primary productivity in both terrestrial and aquatic ecosystems.