If a predator is added to the food web, the population of the prey species would generally decrease, at least initially. The presence of a new predator increases the mortality rate of the prey, leading to a reduction in their numbers. This may cause the prey population to fluctuate depending on various factors such as the availability of food, habitat, and other environmental conditions. Eventually, if the predator population stabilizes, you might see cyclical fluctuations between the predator and prey populations, where the prey population can experience rebounds in numbers when predator populations decline due to food shortages or other factors.
So, the most accurate answer would be that the prey population is likely to decrease initially and may subsequently fluctuate over time.