Final answer:
The Spanish flu pandemic of 1918-1919, caused by an H1N1 virus, was the most lethal influenza epidemic in history, killing an estimated 40 to 50 million people worldwide, which makes it the correct answer (a).
Step-by-step explanation:
The worst influenza epidemic of all time is the Spanish flu, which answers a). Occurring from 1918 to 1919, the Spanish flu, caused by an H1N1 virus with genes of avian origin, claimed an estimated 40 million to 50 million lives globally. This pandemic surpassed the global spread and mortality of other influenza outbreaks such as H1N1 (2009), H3N2, and avian flu (H5N1). The Spanish flu disproportionately affected young adults and spread rapidly due to the conditions of World War I, which included crowding, poor sanitation, and the mobilization of troops and animals.