Which principle applies if an agent performs their existing statutory duty?

The principle that applies when an agent performs their existing statutory duty is that it is not good consideration. In contract law, consideration refers to something of value that is exchanged between parties. For consideration to be valid, it must involve a legal detriment or benefit that is not already owed.

When an agent performs a statutory duty, they are fulfilling an obligation that they are already legally required to perform. Since there is no additional benefit or detriment to the promisee (the party to whom the duty is owed), this performance does not qualify as good consideration. The essence of consideration is that it must involve something new or additional beyond what is already required by law.

In contrast, the other options touch upon different aspects of contractual relationships and obligations, but they do not accurately reflect the principle regarding existing statutory duties. Valid consideration and automatic liability are concepts that relate to entirely different scenarios within contract law, while the notion of not requiring consent is applicable in cases involving unilateral obligations, which does not apply here. Thus, the correct understanding is that the performance of an existing statutory duty does not constitute good consideration.