**Core Concept**
Epidural anaesthesia involves injecting local anaesthetic agents into the epidural space surrounding the spinal cord, providing effective regional analgesia for various surgical procedures. The technique carries inherent risks, including potential complications related to the injection site and surrounding neural structures.

**Why the Correct Answer is Right**
The correct answer is D. acd. Epidural anaesthesia indeed decreases the risk of headache (a) due to the avoidance of puncturing the dura mater, which is a common cause of post-dural puncture headache (PDPH). Additionally, epidural anaesthesia is associated with a lower risk of hypotension (c) compared to general anaesthesia or spinal anaesthesia, as the sympathetic nerves in the epidural space are not directly affected, thus preserving vasomotor tone. However, the risk of meningitis (b) and arachnoiditis (d) is not decreased by epidural anaesthesia; in fact, these complications can occur if the epidural space is infected or if there is a breach of the dura mater.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because hypotension is not decreased by epidural anaesthesia; if anything, it may be exacerbated due to sympathetic nerve blockade.
* **Option B:** This option is incorrect because arachnoiditis is a potential complication of epidural anaesthesia, particularly if the dura mater is inadvertently punctured or if there is an infection.
* **Option C:** This option is incorrect because hypotension is not decreased by epidural anaesthesia; if anything, it may be exacerbated due to sympathetic nerve blockade.

**Clinical Pearl / High-Yield Fact**
When administering epidural anaesthesia, it is essential to use atraumatic needles and to avoid puncturing the dura mater to minimize the risk of post-dural puncture headache (PDPH). Additionally, a thorough understanding of the anatomy and potential complications is crucial to ensure safe and effective epidural anaesthesia.

**✓ Correct Answer: D. acd. Epidural anaesthesia indeed decreases the risk of headache and hypotension but not meningitis and arachnoiditis.**