Final answer:
The primary reason for the massive defeat of native populations during European exploration was the lack of immunity to European diseases such as smallpox, which caused widespread and devastating epidemics.
Step-by-step explanation:
The major contributing factor to the European explorers defeating the natives in mass quantities was c) Immunity to local diseases. While advanced weaponry and technology did play a role, it was primarily the introduction of diseases like smallpox to which the native populations had no immunity that caused devastating outbreaks and massive population declines. Indigenous peoples suffered severe mortality rates as they were exposed to a barrage of new illnesses brought by the Europeans.