Answer:
The individual is able to use critical thought processes to handle a problem.
Step-by-step explanation:
According to Piaget's theory of development, an 18-year old individual would be in the fourth stage of development (Formal Operational Stage). In this stage, Piaget believes that is is no more about changing how knowledge is understood or acquired, but instead this stage deals with building upon already stored knowledge. In this stage, an individual is able to understand abstract concepts. Thus using critical thought process to handle a problem falls under this stage of development.