**Core Concept**
Spinal anesthesia involves injecting local anesthetic drugs into the cerebrospinal fluid (CSF) surrounding the spinal cord to produce sensory and motor blockade. The key principle here is the deposition of the drug within a specific anatomical space to achieve the desired effect.

**Why the Correct Answer is Right**
The correct answer, **B. Pia and arachnoid**, is based on the anatomy of the spinal cord. The pia mater is the innermost layer of the meninges, adhering to the surface of the spinal cord, while the arachnoid mater is the middle layer that surrounds the pia mater. Injecting the anesthetic drug between these two layers allows it to spread throughout the CSF, effectively blocking nerve conduction in the spinal cord. This space is known as the subarachnoid space.

**Why Each Wrong Option is Incorrect**
**Option A:** The dura mater is the outermost layer of the meninges, and injecting the drug between the dura and arachnoid would not allow it to spread effectively, leading to inadequate anesthesia.

**Option C:** The space between the dura and vertebra is the epidural space, where epidural anesthesia is performed. This is a different technique from spinal anesthesia.

**Option D:** Injecting the drug into the cord substance would be highly toxic and would not allow for the desired spread of the anesthetic.

**Clinical Pearl / High-Yield Fact**
Remember that the subarachnoid space is the key to effective spinal anesthesia, and the anatomy of the meninges is crucial for understanding the spread of the anesthetic.

**✓ Correct Answer: B. Pia and arachnoid**