Final answer:
The Seminal Vesicles secret a thick fluid that aids the motility of sperm in the male reproductive system.
Step-by-step explanation:
The anatomic structure of the male reproductive system that secretes a thick fluid aiding the motility of sperm is the Seminal Vesicles (C). The seminal vesicles are a pair of glands that produce a thick, yellowish, and alkaline solution that comprises about 60 percent of the bulk of semen. This fluid is important for reversing the acidity of the vaginal environment, allowing sperm to become motile. The solution contains mucus, fructose, which serves as a nutrient to sperm mitochondria, a coagulating enzyme, ascorbic acid, and prostaglandins. These substances collectively help to support and facilitate the movement of sperm.