Final answer:
The necessary step in preparation for dentin bonding is isolating the tooth with a rubber dam.
Step-by-step explanation:
In preparation for dentin bonding, the necessary step is A) Isolating the tooth with a rubber dam. The rubber dam is a thin sheet that is placed around the tooth to isolate it from the rest of the mouth. This prevents saliva or other contaminants from interfering with the bonding process. By isolating the tooth, the dentist can ensure a clean and controlled environment for successful bonding.