Why was the aboriginal population reduced when european colonist settled in australia a. they brought animals with them b. settled most aboriginal land c. the harsh environment of the outback killed many natives d. they brought diseases with them

The answer is D. They brought diseases.

The Aboriginal people did not have immunity from many of the diseases which have existed in Europe for centuries as before their interaction with the Europeans these diseases were unknown to their shores.  The lack of immunity led to outbreaks which claimed the lives of a significant portion of the Aboriginal population.