Which of the following is considered not good consideration?

Past consideration refers to a situation where something has already been done or provided before a promise is made, and thus it cannot serve as valid consideration for a new contract. In contract law, for consideration to be valid, it must be something that is provided in exchange for the promise at the time the contract is made or in the future. Since past consideration does not involve a bargain struck at the time of the agreement, it is typically not recognized as sufficient consideration to support a new promise or agreement. This is why it is deemed "not good consideration."

In contrast, performance of an existing contractual duty is often seen as consideration in certain contexts, particularly when it goes beyond what is required by the existing contract. Promissory consideration, where a party makes a promise in exchange for a promise from the other party, remains a valid form of consideration as long as it is mutually agreed upon. Paid consideration, or consideration that involves an exchange of payment for goods or services, definitely establishes a valid contractual agreement. Therefore, these other concepts do not share the same issues as past consideration, confirming why past consideration is specifically categorized as not good consideration.