Choose the sentence in which the commas are placed correctly. If you think a phrase in a sentence is non-defining (that is, it could be plucked out of the sentence), then you place commas around it.

O My sister, who is a doctor, is afraid of dogs.
O My sister who is a doctor is afraid of dogs.
O My sister, who is a doctor is afraid of dogs.
O My sister who is a doctor is afraid of dogs.