**Core Concept**
Semen is a complex mixture of fluids from various glands in the male reproductive system. The composition of semen is essential for fertilization and the overall health of the reproductive system.

**Why the Correct Answer is Right**
The majority of semen is contributed by the seminal vesicles, which are a pair of glandular structures located behind the bladder. The seminal vesicles produce a significant portion of the seminal fluid, which is a nutrient-rich, alkaline secretion that helps to neutralize the acidity of the vagina and create a suitable environment for sperm to survive. The seminal vesicles' secretions account for about 60-70% of the total semen volume.

**Why Each Wrong Option is Incorrect**
**Option A:** Ejaculatory ducts contribute to the semen, but they are not the primary source of the majority of semen. They are responsible for transporting the secretions from the seminal vesicles and prostate gland to the urethra.
**Option B:** The prostate gland contributes a smaller amount of semen compared to the seminal vesicles. While it is essential for the production of prostatic fluid, which helps to nourish the sperm, it does not produce the majority of semen.
**Option C:** The bulbourethral glands, also known as Cowper's glands, produce a small amount of clear fluid that helps to lubricate the urethra and facilitate the passage of semen. However, they do not contribute significantly to the overall volume of semen.

**Clinical Pearl / High-Yield Fact**
The seminal vesicles' secretions are rich in fructose, which serves as an energy source for sperm. This is a critical aspect of male fertility, as sperm require a constant supply of energy to maintain their motility and viability.

**Correct Answer:** C. Prostate gland.