Answer: C. Producer populations will decrease, causing a decrease in consumer populations.
Step-by-step explanation:
According to the situation, the volcanic ash will mask the light energy from sun. The sunlight is the main requirement for the producers to synthesize their own food by the process of photosynthesis. The population of plants and other autotrophs will be affected by this. The other organisms in the ecosystem will be affected as decline in the population of producers will result in decline in the population of consumers due to lack of food.