When primary consumers (herbivores) disappear from an ecosystem, the immediate effect is typically that the population of producers (plants) increases, as there are no primary consumers to eat them. This can lead to an abundance of vegetation. However, the secondary consumers (carnivores) that rely on primary consumers for food will likely decrease in population due to the lack of available food sources.
Therefore, the most accurate response is:
The producer population increases, while the secondary consumer population decreases.