Final answer:
The prostate gland secretes an alkaline substance that constitutes about 30% of the seminal fluid.
Step-by-step explanation:
The gland that secretes an alkaline substance that constitutes about 30% of the seminal fluid is the prostate gland. The prostate gland is a walnut-sized organ located below the seminal vesicles and it surrounds the urethra. It secretes a slightly alkaline fluid that makes up close to 30 percent of the total volume of semen, which helps maintain sperm quality and motility.