Final answer:
Mature adult sexuality in Freud's theory of psychosexual development develops during the d)genital stage, which begins at puberty.
Step-by-step explanation:
In Freud's theory of psychosexual development, mature adult sexuality develops during the d) genital stage. This is the final stage of development, beginning at puberty when sexual feelings reawaken. According to Freud, individuals in the genital stage redirect their incestuous urges towards socially acceptable partners and if this stage is successfully completed without any fixations from earlier stages, the individual is considered to be a well-balanced, healthy adult. Earlier stages include the oral, a-n-a-l, and phallic stages, followed by a latency period where sexual impulses are dormant as the child focuses on developing in other areas of life.