Which of the following is an example of a contract obligation?

A contract obligation refers to the responsibilities or duties that each party is required to fulfill as part of the agreement. In the context of the options provided, the most fitting example of a contract obligation is the payment for services rendered. This is a clear and direct action that one party must perform in accordance with the terms laid out in the contract. When a service provider completes their work, the obligation of the party receiving those services is to compensate them as agreed upon.

The other options engage with processes or considerations surrounding contracts, but they do not exemplify an obligation in the same direct manner. Negotiating contract terms is a preliminary stage that occurs before the contract is finalized and does not represent an obligation since no agreement has yet been reached. Changing contract prices may alter the existing obligations, but it is not an obligation in itself; it's more of a modification. Similarly, extending contract terms involves a discussion about extending the timeframe of obligations but does not denote a specific obligation being enacted. Thus, the obligation of payment for services rendered stands out as a definitive example of what one party is contractually required to do.