**Core Concept**
The substantia gelatinosa is a critical structure within the spinal cord's dorsal horn, playing a key role in the processing and transmission of pain signals. It is part of the laminated structure of the spinal cord and is involved in the modulation of pain perception.

**Why the Correct Answer is Right**
The substantia gelatinosa is situated in the outermost layer of the spinal cord's dorsal horn, specifically within the dorsal root entry zone. This region is rich in unmyelinated and thinly myelinated nerve fibers that transmit pain and temperature sensations. The substantia gelatinosa contains neurons that receive and process these pain signals, and it is also the site of action for various analgesic substances, including opioids and local anesthetics. The gelatinous appearance of this region under the microscope is due to the abundance of neuroglial cells and the loose arrangement of its neurons.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the substantia gelatinosa is not a structure within the ventral horn of the spinal cord, which is primarily involved in the transmission of motor signals.

**Option B:** This option is incorrect because the substantia nigra is a structure located in the midbrain, not the spinal cord, and is primarily involved in motor control and reward processing.

**Option C:** This option is incorrect because the posterior horn of the spinal cord is primarily involved in the transmission of proprioceptive and sensory information, but it does not specifically correspond to the substantia gelatinosa.

**Clinical Pearl / High-Yield Fact**
The substantia gelatinosa is a critical target for various analgesic substances, including opioids, which act by binding to mu-opioid receptors on neurons within this region. This binding inhibits the release of pain-transmitting neurotransmitters, providing pain relief.

**Correct Answer: C. Dorsal horn of the spinal cord.**