Final answer:
The additive used to thicken jellies and jams is Pectin, a natural polysaccharide found in fruit that acts as a gel-forming substance in these spreads.
Step-by-step explanation:
The additive used to thicken jellies and jams is A) Pectin. Pectin is a natural polysaccharide found in fruit juices and is a critical ingredient for creating the gel-like consistency that is characteristic of jellies and jams. It acts as a gel-forming substance due to its ability to form a colloidal system when combined with sugar and acid during the cooking process. Pectin is distinct from gelatin, which is a protein-based gelling agent used in desserts like Jell-O, or xanthan gum and cornstarch, which are used as thickeners and stabilizers in various cooking and baking applications.