If this sentence is changed to passive voice, then the subject is also changed, in order to retain the meaning of the sentence.

So, the object takes the place of the subject and vice versa and the word 'by' is introduced as now the subject receives the action by the object. The verb form of the sentence is also changed from active form to passive.

The subject here is a noun phrase.

'Subject (The noise of the traffic) + verb (kept) + object (me) + object complement (awake)' changes to:

'New subject (I) + verb (was kept) + object complement (awake) + by + new object (the noise of the traffic)'

The passive form of 'kept' in the simple past tense is 'was kept'.

This can be seen in option A, hence it's the right answer.

Option B: The subject here has been split into two parts and placed before and after the verb 'kept'. As a result, the meaning of the sentence has changed.

Option C: The subject here 'I' is performing the action towards itself. The meaning of the sentence has hence changed.

Option D: Here the verb 'kept' has been replaced by 'remained', which is not called for.