Step 1: Define the concept.
{Social closure} (Weber, later Parkin) describes strategies by which an in-group $\Rightarrow$ limits entry and monopolizes valued resources (e.g., credentials, licenses, networks) to maintain advantage.
Step 2: Eliminate distractors.
(A) {Social action} is a broad term for meaningful action oriented to others.
(B) {Social construction} concerns how meanings/reality are produced.
(D) {Social conflict} is struggle over resources but not specifically about restricting membership.
Therefore (C) matches the definition.