D

Indirect retainers are rests (or similar rigid components) placed anterior to the fulcrum line of a distal-extension RPD; when sticky food or gravity tries to lift the free-end base away from the ridge, they counteract that rotational, occlusal-direction displacement, thereby preventing dislodgement of the extension base [Frank & Nicholls, 1977, PMID 335050; McDowell & Fisher, 1982, PMID 7040637].