An effective way to make language more emphatic when using English is by inverting a sentence's normal word order.
For example, 'I have never eaten such a flaky croissant.' is a regular sentence, with the word order: subject (I) + verb (have never eaten) + object (such a flaky croissant).
To add emphasis, the auxiliary verb comes before the subject: Never have I eaten such a flaky croissant. ('Never' is an adverb and the main verb is 'eaten')
The given sentence has also altered the normal word order, verb (have) + subject (I) +main verb (witnessed), hence it requires no change and option A is correct.
Option B is incorrect because the word order in the sentence is normal, i.e., subject + verb
Option C is incorrect because only the adverb 'never' is altered, the verbs 'have witnessed' are in the normal order.
Option D is incorrect because the adverb here 'never' is placed before the auxiliary, whereas it is usually placed after the auxiliary verb, 'have' (in the normal order).