Final answer:
A fixation is defined as an unresolved conflict caused by overindulgence or frustration, often stemming from one's developmental stages, according to Freud's psychoanalytic theory (option d).
Step-by-step explanation:
A fixation in psychological terms is best defined by option (d): an unresolved conflict caused by overindulgence or by frustration. This concept originates from Sigmund Freud's psychoanalytic theory, where he suggested that if an individual's needs are not properly met during a specific stage of development, they may become 'fixated,' leading to certain psychological patterns or behaviors into adulthood. For example, an adult with an oral fixation may indulge in overeating or excessive alcohol consumption as a result of issues stemming from the oral stage of development.
Freud's theory of psychosexual development outlines several stages, such as the oral, rectal, and phallic stages. Emotional and psychological consequences throughout adulthood may be the result of a failure to navigate these stages properly. The idea of fixations continues to inform various disciplines, despite a lack of empirical evidence backing Freud's entire theory.
Hence, the answer is option d.