Final answer:
The correct answer is option a. The most common disease known to be carried by fleas is the bubonic plague, caused by the bacterium Yersinia pestis, with high mortality if untreated. It historically led to pandemics like the Black Death.
Step-by-step explanation:
Fleas are known vectors for various human diseases, but the most common disease associated with them is plague. Specifically, the bubonic plague, which is caused by the bacterium Yersinia pestis. Historically, plague-bearing fleas have transmitted this disease to humans after their usual rodent hosts have died from the infection.
There are three forms of plague: bubonic, pneumonic, and septicemic, with bubonic being the most common. The disease is known for causing swellings or buboes, and it had devastating effects during pandemics such as the Black Death. In contrast, diseases like malaria and yellow fever are transmitted by mosquitoes, not fleas.