**Core Concept:** Alignment refers to the process of restoring the position of a tooth or a group of teeth to their normal position in the dental arch. In orthodontics, various wires are used for this purpose based on their shape, flexibility, and material composition.

**Why the Correct Answer is Right:**
The correct answer, **Correct Answer: D.** is a nickel-titanium (NiTi) wire. Nickel-titanium wires are made of superelastic NiTi alloy. They exert compressive forces when heated, and tensile forces when cooled, which allows them to align teeth efficiently. The shape memory effect of NiTi wires enables them to return to their original shape after being bent, reducing the need for frequent wire replacements.

**Why Each Wrong Option is Incorrect:**
A. **Option A:** Stainless steel (SS) wires are commonly used in fixed orthodontic appliances. However, they exhibit tensile forces only and are generally softer than NiTi wires, making them less efficient for aligning teeth.

B. **Option B:** Copper-nickel-tin (CuNiTi) wires are similar to NiTi wires in terms of shape memory effect but have lower shape memory temperature and are less effective in aligning teeth compared to NiTi.

C. **Option C:** Tungsten carbide wires are rigid and inflexible, making them unsuitable for aligning teeth due to their inability to exert the required compressive forces.

**Clinical Pearl:** Superelastic NiTi wires are a superior choice for orthodontic alignment due to their unique properties, including compressive forces, shape memory effect, and higher shape memory temperature compared to CuNiTi wires.