Explanation:

The correct answer is option D - Mammary gland.

Secondary sex organs are organs that are involved in reproduction and are differentiated between males and females. These organs are not directly involved in the production of gametes (sperm or eggs), but they play a crucial role in the reproductive process.

Here is a detailed explanation of each option:

a) Vagina:
The vagina is a part of the female reproductive system and is considered a secondary sex organ. It is a muscular canal that connects the uterus to the external genitalia. During sexual intercourse, the vagina serves as the entry point for the penis and also acts as the birth canal during childbirth.

b) Penis:
The penis is a part of the male reproductive system and is considered a secondary sex organ. It is responsible for the delivery of sperm into the female reproductive tract during sexual intercourse.

c) Prostate:
The prostate gland is a part of the male reproductive system and is considered a secondary sex organ. It is located just below the bladder and surrounds the urethra. The prostate gland produces a fluid that mixes with sperm to form semen.

d) Mammary gland:
The mammary gland is not a secondary sex organ. It is a glandular organ in the breasts of females and is responsible for the production of milk. While the mammary gland is essential for nourishing offspring after birth, it is not directly involved in the reproductive process and is therefore not considered a secondary sex organ.

In conclusion, the mammary gland is not considered a secondary sex organ because it is not involved in the reproductive process. The other options, including the vagina, penis, and prostate gland, are all secondary sex organs as they play crucial roles in reproduction.