Final answer:
When Dad D dies intestate, his estate will be distributed equally among his three surviving children, regardless of the $25,000 gift given to son A.
Step-by-step explanation:
When Dad D dies intestate, meaning without a will, the distribution of his estate will follow the state intestacy laws. In this case, Dad D has three surviving children: son A, daughter B, and son C. The estate will be distributed equally among all three children, which means each child will receive one-third of the estate. The $25,000 gift given to son A on his 25th birthday will not affect the distribution of the estate.