When primary consumers disappear from an ecosystem, the expected effect on the populations of producers and secondary consumers would typically be:
The producer population increases, while the secondary consumer population decreases.
This is because primary consumers (herbivores) feed on producers (plants). If primary consumers are removed, the producer population would likely increase due to a lack of grazing pressure. Meanwhile, secondary consumers (carnivores) that rely on primary consumers for food would decrease as their food source is diminished.
So the correct response is: The producer population increases, while the secondary consumer population decreases.