In a freshwater ecosystem, the primary consumer is typically an organism that feeds directly on producers, such as plants or algae. Among the options provided:
a. Duckweed - This is a producer as it is a type of aquatic plant. b. Zooplankton - This is a primary consumer, as they feed on phytoplankton (which are producers). c. Dragonflies - These are predators and generally not considered primary consumers.
Therefore, the correct answer is b. Zooplankton.