(b) semen

Explanation:

The prostate gland is a chestnut shaped gland which lies at the base of bladder and surrounds the first part of the urethra. This gland secretes a slightly alkaline fluid which forms the important component of semen. This fluid constitutes the major portion of seminal fluid which carries sperm and sperms move freely in this fluid. Prostate gland secretion contains lipids, small amount of citric acid, HCO^-₃ ions and a few enzymes. They activate and provides nutrition to sperms and neutralise the acidity of urine which may kill the sperms. They form about 30% part of semen.