In brazil, the indigenous population was around 3.5 million before the arrival of europeans. today, the indigenous population is around
a. 500,000
b. 200,00
c 1.5 million

Is there another option? Because the estimates that I know say that it is around 1 million people, which is not an option here.

I would choose 1.5 million: it is likely that the number is too small: some groups are not contacted yet and some people might choose not to identify as indigenous due to stigmatization.