Final answer:
Adolescents concerned with abstract future-oriented concepts like marriage and career are in Piaget's formal operational stage. Angelica's self-consciousness, assuming others are staring at her, is an example of the imaginary audience phenomenon described by Elkind.
Step-by-step explanation:
According to Jean Piaget's developmental theory, adolescents who are concerned with future-oriented concepts such as getting married, having friends, getting a job, and doing well in school are in the formal operational stage of cognitive development. This stage typically begins around age 11 and up. Adolescents in this stage have the ability to think abstractly and deal with hypothetical situations.
David Elkind described adolescent egocentrism, which manifests in behaviors like feeling overly self-conscious. A specific form of this egocentrism is the imaginary audience phenomenon, where an adolescent, like Angelica at her friend's party, believes that others are as focused on them as they are on themselves.