Australia was discovered in the 18th century. It presents a past action.
Option C: 'When was...discovered?' is in simple past tense in passive voice. The sentence is in passive voice since the subject 'Australia' experiences the action 'was discovered'.
The sentence is complete and meaningful, hence option C is correct.
Option A: 'Has been' is in present perfect tense and states an action just completed. It doesn't ask the question 'when', which asks about a past event.
Option B: 'When Australia was discovered' is not in the correct word order.
The correct word order for an interrogative sentence (passive voice) is:
'Question word (when) + helping verb (was) + subject (Australia) + main verb (discovered)?
Option D: 'Has been' is again in the present perfect tense. Also, the word order isn't correct.
Hence options A, B and D are wrong.