Plant tissues contain a diverse array of toxic or potentially toxic substances, such as resins, tannins, glycosides, and alkaloids, many of ________are highly effective deterrents to insects that feed on plants.
句意：植物组织含有多种有毒或潜在的有毒物质，如树脂、单宁、糖苷和生物碱，其中许多都是对以植物为食的昆虫的有效阻碍。which引导定语从句，修饰表示事物的先行词，在定语从句中充当主语。因此，本题的正确答案为C。