The correct answer is option 2: Ronald Ross. Ronald Ross, a British physician, is credited with discovering that mosquitoes served as carriers of malaria. He made this groundbreaking discovery in 1897 while conducting research in India. Ross was able to prove the link between mosquitoes and the transmission of malaria by conducting experiments on birds infected with the disease. He demonstrated that when infected mosquitoes bit the birds, they would also become infected with malaria. This discovery was a significant milestone in the understanding of malaria and paved the way for further research and the development of strategies to combat the disease. It also led to Ross being awarded the Nobel Prize in Physiology or Medicine in 1902 for his significant contribution to the field.