The number of aboriginal inhabitants in Australia is considerable.

Answer: Option A

Primitive : relating to, denoting, or preserving the character of an early stage in the evolutionary or historical development of something.
Modern : relating to the present or recent times as opposed to the remote past.
Unoriginal : lacking originality; derivative.
Irrational : not logical or reasonable.
Aboriginal : inhabiting or existing in a land from the earliest times or from before the arrival of colonists; indigenous.

Synonym of Aboriginal is Primitive