When primary consumers disappear from an ecosystem, the most likely outcome is that both the producer population (plants) will increase and the secondary consumer population (animals that eat primary consumers) will decrease. This is because primary consumers play a crucial role in controlling the population of producers; without them, producers can grow unchecked, leading to an increase in their population. At the same time, the secondary consumers will have less food available (since primary consumers are their food source), leading to a decline in their population.
So the correct response would be:
The producer population increases, while the secondary consumer population decreases.