Final answer:
Coastal estuary lakes are the most likely to be naturally eutrophic due to the mix of nutrients from freshwater and seawater sources.
Step-by-step explanation:
The type of lake that would be most likely to be naturally eutrophic is d) coastal estuary lakes. Eutrophic lakes are nutrient-rich and often high in biological productivity. This is typically the case with coastal estuary lakes, where the freshwater from rivers mixes with the salty ocean water, resulting in nutrient input from both the river and the sea. Glacial lakes are usually oligotrophic because they are often newly formed with low nutrient levels. Oligotrophic lakes, by definition, are nutrient-poor and have low productivity. Artificial reservoirs can be eutrophic if they receive nutrient runoff, but aren't necessarily the most likely to be naturally eutrophic. Coastal estuary lakes, with their mix of nutrients from both fresh and saltwater sources, are often fertile environments for the growth of aquatic plants and algae.