What word is best associated with the idea of an agent or substitute?

The term that aligns most closely with the concept of an agent or substitute is "proxy." A proxy functions as a person or entity that acts on behalf of another, particularly in situations where representation is needed, such as voting or decision-making. This alignment highlights the role of a proxy as a replacement or substitute, executing specific tasks in accordance with the wishes or authority of the party being represented.

While the other terms share similarities with the concept of representation or substitution, they can carry slightly different connotations or be specific to particular contexts. A delegate is often associated with someone chosen to represent a group, particularly in a formal context, while a representative carries the broader implication of someone who embodies the interests of others. A stand-in typically refers to someone who temporarily takes the place of another, often in performance or roles, but may not encompass the formal or legal connotations that a proxy does. Thus, "proxy" is the most fitting term in this context.