D

Non-working-side (balancing) contacts occur between the inner inclines of the supporting cusps (upper lingual or lower buccal). Selective grinding guidelines state that you eliminate the interference by reducing whichever of those supporting cusps is involved and most convenient, so either arch’s supporting cusp may be trimmed—there is no need to shorten both or to touch the non-supporting cusps [Solow, 2005, PMID 16128358].