Role-by-role elimination: A project leader is assigned to head one specific, defined project, not selected broadly for local leadership. A cooperator simply agrees to participate in or support extension activities, without being chosen for leadership qualities. An administrator manages the institutional or organisational side of an extension programme, a desk role rather than a grassroots one.
The description given, someone chosen because of personal interest and fitness to guide some phase of the local extension effort, matches none of these narrower roles but precisely fits the definition of a community leader, a locally respected person mobilised to advance extension work informally.
Eliminating the other three role definitions confirms community leader as correct.