The correct answer is advocate because a proponent is someone who supports or argues in favor of something. An advocate also supports or pleads for a cause or proposal. They both share the same fundamental meaning of promoting or backing an idea or action.
The other options are incorrect: * Predecessor means someone who came before you in a position or role. * Conspirator is someone involved in a secret plan to do something harmful or illegal. * Informer is a person who gives information to the police or other authority about something.