Final answer:
Jack and Jill likely developed a fixation during the phallic stage of psychosexual development, where unresolved conflicts can lead to adult fixations such as vanity or emotional immaturity.
Step-by-step explanation:
In considering which stage of psychosexual development Jack and Jill likely developed a fixation, we must look at Sigmund Freud's stages of development. The phallic stage is the third stage, occurring roughly between the ages of 3 and 6, where the erogenous zone is the genitals.
During this time, boys experience the Oedipus complex, which includes a desire for the mother and feelings of rivalry toward the father. Failure to resolve these conflicts can lead to fixations later in adulthood.
Examples of such fixations can include vanity, ambition, promiscuity, or emotional immaturity. Without knowing further details about Jack and Jill, it can't be said for certain where they might have developed a fixation, but the phallic stage is a common phase where these issues can begin if not properly navigated.