Which of the following gifts made by Tristan, age 49, is subject to the generation-skipping transfer tax?

a) $20,000 to his niece, age 6.

b) Payment to a medical school for tuition of $50,000 for the granddaughter of Joe, Tristans best friend.

c) $20,000 to the daughter (age 6) of Tristans former college roommate.

d) $15,000 to his grandson.