Final answer:
Reg. §25.2503-4 deals with estate taxes, which affect inheritances over $5.43 million as of 2015, applying to a small number of wealthy individuals.
Step-by-step explanation:
The regulation that deals with estate taxes is Reg. §25.2503-4. This regulation is part of the Internal Revenue Code and provides guidelines related to the estate and gift taxes. The United States estate tax is a tax imposed on the value of an inheritance, which suggests a willingness to limit how much wealth can be passed on. As of 2015, the estate tax applied only to estates leaving inheritances of more than $5.43 million, affecting only a tiny percentage of those with high levels of wealth. Hence, the correct answer to the question is option 'b'.