**Core Concept**
The ejaculatory ducts are a pair of short, muscular tubes that play a crucial role in the male reproductive system. They are formed by the union of the vas deferens and the seminal vesicle ducts, and their primary function is to transport sperm and seminal fluid from the epididymis to the prostate gland.

**Why the Correct Answer is Right**
The ejaculatory ducts open into the prostatic urethra, which is a part of the male urethra that passes through the prostate gland. This anatomical arrangement allows the sperm and seminal fluid to mix with the prostatic secretions and then be propelled out of the body during ejaculation. The ejaculatory ducts are lined with smooth muscle and are under the control of the autonomic nervous system, which helps to regulate the contraction and relaxation of the ducts during ejaculation.

**Why Each Wrong Option is Incorrect**
**Option A:** The ejaculatory ducts do not open into the ureter, which is a tube that carries urine from the kidney to the bladder. The ureter is a separate structure that is not involved in the reproductive process.
**Option B:** The ejaculatory ducts do not open into the vas deferens, which is the muscular tube that carries sperm from the epididymis to the prostate gland. The vas deferens and the ejaculatory ducts are two distinct structures that work together to transport sperm and seminal fluid.
**Option C:** The ejaculatory ducts do not open into the seminal vesicle, which is a gland that produces a significant portion of the seminal fluid. While the seminal vesicle does contribute to the formation of the ejaculatory ducts, it is not the site where they open.

**Clinical Pearl / High-Yield Fact**
It's worth noting that the ejaculatory ducts can be a common site for obstruction, which can lead to infertility or other reproductive problems. A condition known as ejaculatory duct obstruction can be caused by a variety of factors, including congenital anomalies, injury, or infection.

**Correct Answer: D. Prostatic urethra**