Answer:
The correct answer is - decreased population size at all levels.
Step-by-step explanation:
A decrease in the productivity of producers of an ecosystem will lead to a decrease in all other organisms of the ecosystem including top-level consumers to the bottom-level producers due to the fact that producers provide energy and food to primary consumers.
It is because they are the only organisms that convert the sunlight to usable energy and food that transfer from one trophic level to other, so if the population of producers or their productivity will decrease then there will be decreased population size at all levels.