Final answer:
a) Sigmund Freud
The description given points to Sigmund Freud's theory of fixation during the psychosexual stages of development, particularly the phallic stage.
Step-by-step explanation:
The description "...eternal adolescents in their erotic life"[who] "become fixated upon the youth and sexual vitality of the adolescent boy" refers to the concept of fixation in the psychosexual stages of development described by Sigmund Freud.
Freud posited that individuals could become fixated at a particular stage if they do not successfully navigate through the psychosexual stages, including the phallic stage where the Oedipus complex arises, and a focus on sexual and genital concerns is prevalent.
The other theorists mentioned, including Carl Jung, Alfred Adler, and Erik Erikson, developed their theories as extensions or responses to Freud’s original work, but they do not specifically focus on erotic fixation in the same way Freud does.