The correct answer is option 4, Ronald Ross. Ronald Ross was a British physician who made significant contributions to the understanding of malaria. In 1897, Ross conducted experiments with mosquitoes and discovered that female Anopheles mosquitoes were responsible for transmitting malaria. Before that, it was not understood how the disease was spread. Ross`s work was groundbreaking and paved the way for further research on the prevention and treatment of malaria. Edward Jenner, option 1, is known for developing the smallpox vaccine. Louis Pasteur, option 2, is known for his work on pasteurization and vaccines. Robert Koch, option 3, is known for his work on germ theory and discovering the bacteria that cause diseases like tuberculosis and cholera.