**Core Concept**
Spinal anaesthesia can cause hypotension due to vasodilation and decreased sympathetic tone, leading to a significant decrease in blood pressure. This is because the sympathetic nervous system, which is responsible for maintaining blood pressure, is blocked by the local anaesthetic.

**Why the Correct Answer is Right**
The most effective method to prevent hypotension during spinal anaesthesia is the administration of crystalloid fluids, specifically lactated Ringer's solution. This is because the fluid helps to increase the intravascular volume, which in turn helps to maintain blood pressure. The administration of colloid fluids, such as albumin, may also be beneficial in some cases.

**Why Each Wrong Option is Incorrect**
**Option A:** Administering vasopressors, such as phenylephrine, may help to increase blood pressure but is not the most effective method of preventing hypotension. Vasopressors can cause tachycardia and are not a substitute for adequate fluid resuscitation.
**Option B:** Elevating the head of the bed may help to reduce venous return and worsen hypotension. It is not an effective method of preventing hypotension during spinal anaesthesia.
**Option C:** Administering a vasodilator, such as nitroglycerin, would exacerbate hypotension and is not an effective method of preventing it.

**Clinical Pearl / High-Yield Fact**
It is essential to administer crystalloid fluids before and during spinal anaesthesia to prevent hypotension. This is a key principle in the management of spinal anaesthesia and can help to reduce the risk of complications.

**Correct Answer: C. Administering crystalloid fluids, specifically lactated Ringer's solution.**