Final answer:
The natural gelling substance found in most fruits is pectin, which is used to thicken jams and jellies when cooked with sugar and acid.
Step-by-step explanation:
The natural gelling substance found in most fruits is pectin. Pectin is a type of polysaccharide that is present in the cell walls of plants and helps to give structure to fruit. When fruits are cooked, pectin is released, which then gels in the presence of sugar and acid, giving jams and jellies their thickness. This process is commonly used in home canning and commercial jam production. Unlike pectin, gelatin is derived from animal collagen, agar is extracted from algae, and carrageenan is also a seaweed derivative, but none of these are naturally found in fruits like pectin is.