Final answer:
The situation that lacks consideration is when a supervisor promises a bonus to the best salesperson but does not pay due to budget cuts, as the promise was not made with the knowledge of the salesperson, and no exchange of value was agreed upon beforehand.
Step-by-step explanation:
Lack of Consideration in Contracts
The situation that lacks consideration from the given options is: after looking at sales figures for the last month, a supervisor promises a bonus to the best salesperson but due to budget cuts never pays the bonus. Consideration is a fundamental element of a contract which refers to the value promised in exchange for a performance or a promise of performance. It is what each party brings to a contract as the 'price' for the agreement. The examples of the two students exchanging books and the customer buying an antique watch involve a clear exchange of consideration. However, a supervisor's retrospective promise of a bonus without the salesperson having been aware of this beforehand, and therefore, not having relied upon this promise in their sales efforts, does not constitute a valid consideration.
In the case of the 17-year-old buying a car, the lack of consideration is not apparent; instead, the functionality of the contract might be in question due to the minor's age, which could potentially allow them to void the contract. It's important to note that each situation should be evaluated in the context of the contract law principles such as mutual consideration, the capacity to contract, and the legality of the agreement.