Final answer:
A smaller quantity of the drug is needed to achieve anesthesia using the subarachnoid block compared to the epidural block due to the closer proximity to the central nervous system, allowing for a more direct delivery of the drug.
Step-by-step explanation:
The correct answer is a) Smaller. A subarachnoid block involves an injection of medication into the cerebrospinal fluid in the subarachnoid space, which is closer to the spinal cord and brain than the space targeted by the epidural block. Because of the proximity to the central nervous system structures that need to be anesthetized, the concentration of the drug can be more directly delivered, and thus a smaller quantity of the drug is required to achieve the desired effect of anesthesia.
In contrast, an epidural block is administered into the epidural space, which is outside the dura mater (the outermost membrane around the spinal cord). As the drug has to diffuse through more tissue to reach the nerves to cause anesthesia, a greater quantity of medication is generally needed when compared to the subarachnoid block.