Answer: B. The population of prey would decrease.
Food chain is a liner sequence of supply of food and energy in an ecosystem, when one organism is consumed by another organism. A food web is an interconnection of the multiple food chains. A predator is an organism in the ecosystem which obtains it's food by killing other organisms. A prey is an organism which is killed by another organism so as to obtain it as food.
In the given situation, if a major predator was added to the food web, the population of the prey will decreased. This is because predator population is dependent upon prey population as a source of food.