The hypothesis of Ronald Ross relating to malaria was:

Explanation

Ronald Ross's hypothesis and subsequent research demonstrated that mosquitoes, particularly the Anopheles species, are vectors for the Plasmodium parasite, which leads to malaria. This was a groundbreaking discovery because it identified the mosquito as the carrier rather than attributing the disease to environmental factors like bad air, as suggested by the miasma theory. The other options incorrectly focus on outdated or already known aspects of malaria transmission, rather than Ross's specific contribution.