Answer:
B. Kim will win, because the bonus is a reward for work they have already performed, which is past consideration and cannot be used to create a contract.
Step-by-step explanation:
In order for a contract to be enforceable, consideration must be exchanged between both parties. In this case, Kim made a promise that included consideration ($3,500) but Gold didn't exchange of give anything back. The swimming pool is already finished and it represents another different contract.
Another example would be a boss telling a subordinate that he/she will receive a bonus for having worked 10 years in the firm. The employee already got paid for working the 10 years, so there is no actual exchange of new consideration.