Final answer:
The thickening agent of a tomato sauce is pectin, a carbohydrate that serves as a stabilizer, gelling agent, and emulsifier, commonly found in fruit juices like those of tomatoes.
Step-by-step explanation:
The thickening agent of a tomato sauce is pectin, which is a carbohydrate found in fruit juices. It acts as a stabilizer, gelling agent, and emulsifier, important in the jelly-making process. Unlike other options such as cornstarch and flour, which could also thicken a sauce, pectin is specifically associated with fruits and is commonly used in tomato sauces due to tomatoes' natural pectin content.
Other substances like gelatin, typically derived from animal sources, is another type of gelling agent, but it is not used as a thickening agent in tomato sauce. Pectin not only thickens but also helps in maintaining the acidity level, making it a versatile acidity regulator as well.