**Core Concept**
The pelvic splanchnic nerves are a group of nerves that arise from the sacral spinal cord and play a crucial role in the autonomic innervation of the pelvic organs, including the bladder, rectum, and reproductive organs. These nerves are primarily involved in the parasympathetic regulation of pelvic organ function.

**Why the Correct Answer is Right**
The pelvic splanchnic nerves are formed by the ventral rami of the S2-S4 spinal nerves, which merge to form a complex network of nerve fibers. These nerve fibers then synapse in the pelvic ganglia, from which postganglionic fibers innervate the pelvic organs. The parasympathetic stimulation of these organs results in increased contractility, secretory activity, and blood flow.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the pelvic splanchnic nerves are not formed by the lumbar spinal nerves.
**Option B:** This option is incorrect because the pudendal nerve, while also involved in pelvic organ innervation, is primarily a somatic nerve and is not responsible for the formation of the pelvic splanchnic nerves.
**Option C:** This option is incorrect because the hypogastric nerve, also known as the sympathetic trunk, is involved in the sympathetic innervation of the pelvic organs, not the parasympathetic innervation.

**Clinical Pearl / High-Yield Fact**
The pelvic splanchnic nerves are often damaged in pelvic surgeries, such as hysterectomies or prostatectomies, leading to potential complications such as urinary retention or fecal incontinence.

**Correct Answer: D. S2-S4 spinal nerves**