Final answer:
Lucas, being four years old, is in the phallic stage of psychosexual development, characterized by the Oedipus complex, where he may show rivalry towards his father for his mother's affection.
Step-by-step explanation:
Four-year-old Lucas stands between his parents when he sees his father trying to kiss his mother. Lucas is going through the phallic stage of development. The phallic stage is the third stage of Sigmund Freud's psychosexual development theory, occurring approximately between the ages of 3 to 6 years. During the phallic stage, the erogenous zone for children is the genitals, and they begin to develop a subconscious desire for the opposite-sex parent while feeling rivalry towards the same-sex parent. For a boy like Lucas, this manifests as the Oedipus complex, marked by a desire for his mother's affection and an unconscious jealousy of his father. The resolution of this complex occurs as the child begins to identify with the same-sex parent. This stage is crucial for personality development, and failure to resolve these conflicts can lead to fixation and traits such as vanity and high ambition.