Which example demonstrates an adjectival suffix?

An adjectival suffix is used to transform a word into an adjective, often providing descriptive qualities or characteristics. In the option "careful," the suffix "-ful" is added to the root word "care." This suffix conveys the meaning of being full of care or concern, thereby describing a quality of a person or thing. Thus, "careful" is an adjective that indicates a cautionary or considerate nature.

The other choices do not illustrate adjectival suffixes effectively. For instance, "invent" derives from the root "vent" without adding an adjective form. "Worker," derived from "work," turns a verb into a noun representing a person who works. Similarly, "act" comes from "argue" but does not add an adjectival quality to the root. Therefore, "careful" is the clear example of a word that has been modified with an adjectival suffix to convey a specific trait.