The given sentence is in the passive voice, as the verb form 'being seen' is passive, it means that the subject 'the thief' will receive the action 'without being seen'.

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

The active form of 'being seen' in the simple past tense is 'seeing'.

Also, the subject (doer) 'anyone' is implied in the passive voice: without being seen by anyone'.

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

Option A: The sentence has been changed to passive voice, where the subject 'the wall' receives the action 'was climbed over'. But, the sentence has to be changed to active voice.

Option B: The verb form 'was climbed over' is in passive voice whereas it needs to be changed to active.

Option C: This sentence is still in passive voice 'without being seen' is the passive verb form.