What is the term for the expiration of an agency if no specific time is defined?

The term "Expiration of time" is the appropriate choice in this context because it refers specifically to the conclusion of an agency relationship that occurs when no specified duration is outlined. In agency law, if an agency agreement does not stipulate a specific time period, it generally endures until the purpose for which the agency was created has been fulfilled or until one of the parties decides to terminate it. However, since an undefined agency is inherently open-ended, it typically comes to an end when the parties agree on their relationship's conclusion or when it is legally or practically terminated.

In contrast, "Termination by law" refers to situations when an agency ends due to legal reasons such as the death of the agent or principal. "Mutation of agreement" is not a standard term in agency law and could be misleading, while "Annulment of contract" pertains to voiding a contract from the outset due to certain factors, which does not apply to agency relationships ending by default or absence of time specification. Thus, the correct answer accurately reflects the context of agency expiration without a defined timeframe.