Which of the following identifies a legal duty in tort law?

In tort law, a legal duty is defined as an obligation that arises from laws or statutes designed to protect individuals and their rights. The correct answer identifies this obligation established by statute, meaning that it is backed by legal authority and can be enforced through the judicial system. This legal duty requires individuals to act in a certain way to avoid causing harm to others, and failing to fulfill this duty can lead to liability for any resulting damages.

This contrasts with the other options: a moral responsibility does not have legal enforcement; social norms, while they can influence behavior, lack the formal legal backing; and informal agreements, although they may indicate some level of responsibility, do not constitute a legal obligation unless they meet specific legal requirements. Therefore, the obligation established by statute is the foundation of legal duties in tort law, making it the correct choice.