Which hairpin-like structure serves to anchor and stabilise the tick's mouthparts to its host's skin and to channel the exchange of fluids?

The correct answer is Hypostome.

Step-by-Step Explanation:
1. Understanding Tick Mouthparts: Ticks are parasitic arachnids that feed on the blood of their hosts. To facilitate feeding, they possess a specialized structure at the anterior end of their body, known as the capitulum (or gnathosoma), which contains their feeding apparatus.
2. Identifying the Anchor Structure: Among the mouthparts, the central, rod-like or hairpin-like structure that is inserted directly into the host's skin is the hypostome. The hypostome is covered with backward-pointing barbs or denticles. These barbs act like anchors, preventing the tick from being easily removed once it has embedded itself.
3. Fluid Exchange: In addition to providing stabilization and anchorage, the hypostome contains a groove or channel that allows the tick to inject saliva (which contains anticoagulants and anesthetics) into the host and to suck blood back into its digestive tract.
4. Evaluating the Options:
- Substratum: This refers generally to the surface or ground on which an organism lives, not an anatomical structure of a tick.
- Peduncle: This is a stalk-like supporting structure found in various plants and animals, but is not the anchoring mouthpart of a tick.
- Trunk: This is a general term for the main body section of an animal, not a specialized cranial feeding structure.
- Hypostome: This is the correct anatomical term for the barbed, stabilizing, and fluid-channeling mouthpart.