- Primary productivity is the rate at which energy is converted by photosynthetic organisms into organic matter. Ecosystems with high resource availability (light, water, nutrients) and favorable temperatures tend to have high productivity. - Comparing the given ecosystems in terms of typical net primary productivity (NPP): - Mangroves: Are among the most productive ecosystems globally, comparable to tropical rainforests, due to high light, nutrient input from tides/rivers, and warm temperatures. (Very High) - Grasslands: Productivity varies significantly depending on rainfall and temperature (temperate vs. tropical grasslands), but is generally moderate. (Moderate) - Lakes: Productivity varies greatly (oligotrophic to eutrophic), but averages around moderate, often higher than open oceans. (Moderate) - Oceans: The vast open ocean is primarily nutrient-limited, resulting in very low average productivity. Coastal areas, estuaries, and upwelling zones are exceptions, but the sheer size of the oligotrophic open ocean dominates the global average. (Low) - Ordering them in decreasing productivity: Mangroves > Grasslands ≈ Lakes > Oceans. Option C fits this general order: Mangroves (highest), Grasslands, Lakes, Oceans (lowest).