Final answer:
The prostate gland is responsible for producing a secretion that is thick, milky, and alkaline which contributes to semen composition. The seminal vesicles and bulbourethral gland also contribute to the semen, but have different roles within the overall composition of semen.
Step-by-step explanation:
The male gland that produces a secretion that is thick, milky, and alkaline is the prostate gland. The seminal vesicles and the bulbourethral gland also contribute to the composition of semen, but they are not responsible for the characteristics described in the question. The seminal vesicles contribute a thick, yellowish and alkaline solution which accounts for about 60 percent of the semen's volume, whereas the prostate gland adds a fluid making up close to 30 percent of semen. Sperm are produced in the seminiferous tubules and make up only 5 percent of the final semen volume. Spermatogenesis, or the production of sperm, takes place in the seminiferous tubules, not the prostate gland.