Option 1: DDT is a pesticide that is used to kill mosquitoes, but it is not considered a biological control method. It is a chemical control method that can have harmful effects on the environment and human health.

Option 2: Gambusia, also known as mosquito fish, is a type of fish that feeds on mosquito larvae. This makes it an effective biological control method for mosquitoes. By introducing Gambusia into water bodies, such as ponds or lakes, the population of mosquito larvae can be reduced.

Option 3: Oil can be used to suffocate mosquito larvae by creating a film on the water surface, preventing them from accessing oxygen. However, this method is not as effective or commonly used as the introduction of biological control agents such as Gambusia.

Option 4: Ointments are typically used to repel mosquitoes from biting humans, but they are not a method of biological control.

Overall, option 2 - Gambusia is the correct answer as it is a biological control agent that feeds on mosquito larvae.