Which of the following is invalid consideration?

In this context, "consideration" refers to something of value that is exchanged between parties in a contract, which is essential for the contract to be enforceable. When evaluating which example represents invalid consideration, one must consider the underlying principles of contract law.

Performing what one is obligated to do does not constitute valid consideration because it is essentially fulfilling an existing duty or obligation. For consideration to be valid, it must involve a new contribution or detractor of value beyond what is already legally required. If a party is already bound by a contract to perform a specific act, then carrying out that act cannot be considered new or additional value being exchanged between the parties. Thus, it fails to meet the necessary elements of valid consideration.

In contrast, a promise to refrain from drinking can be seen as valid consideration because it involves giving up a legal right. Similarly, a promise for a future act is valid because it represents a commitment to do something in the future, introducing an element of future value into the agreement. Finally, payment in exchange for services rendered is a classic example of valid consideration, as it involves an exchange of services for monetary compensation, both offering tangible value to the parties involved.