Which of the following best defines a breach of contract?

A breach of contract occurs when one party fails to fulfill their obligations as outlined in an agreement, without a legal justification. This definition captures the essence of what constitutes a breach, emphasizing the importance of adhering to the commitments made in a contract.

In the context of this definition, the failure to perform a promise indicates a neglect to meet the terms set forth during the creation of the contract, which can lead to legal repercussions for the offending party. Understanding a breach in this manner is critical, as it lays the groundwork for potential remedies, such as damages or specific performance, which can be pursued by the aggrieved party.

The other options reflect different aspects of contract law but do not describe a breach accurately. Performing contract obligations refers to fulfilling the terms of an agreement, which is not a breach. Negotiating contract terms pertains to the process of reaching an agreement, rather than a failure to perform. Modifying an existing contract involves changing the terms and does not inherently indicate a breach has occurred unless the modification is contested or leads to unfulfilled obligations. Hence, the emphasis on failure to perform without a legal excuse is what correctly captures the definition of a breach of contract.