Which of the listed options is not part of the mammalian male reproductive organ.

The question asks which of the listed options is not part of the mammalian male reproductive organ.

• A) The epididymis is a coiled tube located at the back of each testis, where sperm mature and are stored. It is part of the male reproductive system.
• B) The vas deferens (also called sperm duct) is a tube that transports sperm from the epididymis to the ejaculatory ducts. It is part of the male reproductive system.
• C) The testis (plural: testes) is the primary male reproductive organ, responsible for producing sperm and male hormones like testosterone. It is part of the male reproductive system.
• D) The vulva refers to the external female genitalia. It is not part of the male reproductive system.

Therefore, the vulva is not a part of the mammalian male reproductive organ.

The final answer is $\boxed{\text{D}}$