Answer: A.) The teeth were examined by the dentist.
Step-by-step explanation:
When using Passive voice, the focus of the sentence becomes the person or thing that is receiving attention not the person or thing that is giving the attention. For instance, The phone is being charged by the salesperson. The salesperson is not the subject or focus of the sentence but rather the phone is.
In option A, the Teeth are the ones receiving the attention and yet are the focus of the sentence as well. This makes this sentence a Passive sentence.