Final answer:
The estate planning mechanism in this scenario is a qualified disclaimer, which allows a person to refuse an inheritance or gift without incurring tax liability.
Step-by-step explanation:
The estate planning mechanism in this scenario is a qualified disclaimer. A qualified disclaimer is a legal tool that allows a person to refuse an inheritance or gift, transferring the asset to another person without incurring any gift or estate tax liability. In this case, Murdoch wrote a letter to his mother, indicating his refusal to accept the gifted stock, and returned the stock certificates. By doing so, he effectively disclaimed the gift.