Which Agreement Is Unenforceable Because of Lack of Valid Consideration

Which agreement is unenforceable because of lack of valid consideration?

A) Johnson agreed to donate $1,000 to the United Hospital Building Fund. Based upon this and similar promises, construction was started.
B) Davis promised to give his neighbor's young son $50 if the boy would refrain from trespassing on his property. Because of Davis's promise, the boy complied.
C) Boyd agreed to sell his used motorboat to Yates for $300. Yates tendered the $300.
D) Philips promised his nephew $5,000 if the boy would refrain from smoking until he reached twenty-one years of age. Because of this promise, the nephew complied.