Final answer:
The prostate gland surrounds the upper portion of the male urethra, secretes fluids that help thicken and then liquefy semen, and is part of the urogenital tract shared by the urinary and reproductive systems. Therefore, the correct option is D.
Step-by-step explanation:
The structure that surrounds the upper portion of the male urethra is the prostate gland. The prostate is a walnut-sized gland located at the base of the bladder, anterior to the rectum, and it encircles the prostatic urethra. Its main function is to secrete an alkaline, milky fluid that is added to seminal fluid to become semen. This fluid plays a critical role in the coagulation and later decoagulation of semen after ejaculation. The temporary thickening of semen helps to retain it within the female reproductive tract, aiding in fertility. The prostate gland is also related to the urogenital tract shared by the urinary and reproductive systems.