Final answer:
The Mississippian culture ended largely due to European-introduced diseases such as smallpox and influenza. The Native Americans had no inherited immunity to these diseases, which caused rapid and devastating population declines.
Step-by-step explanation:
The end of the Mississippian culture (option a) was caused chiefly by diseases such as smallpox and influenza brought by the Europeans, to which the Native Americans had no immunity. The Mississippian culture thrived until around 1375, after which it declined and evolved into smaller distinct chiefdoms by the time Europeans made contact. The indigenous populations across the Americas experienced dramatic declines due to infectious diseases introduced by European contact, which spread rapidly among communities with no prior exposure, wreaking havoc on their societal structures and often leading to the collapse of large chiefdoms as seen in the Mississippian culture's case.