Final answer:
Nickel-titanium arch wires are favored in early orthodontic treatment due to their high flexibility and elasticity. Stainless steel, while used in orthodontics, is less flexible, and other materials like gold and copper have different properties that may not make them as suitable for this specific application.
Step-by-step explanation:
Arch wires that are made of nickel-titanium are useful in early stages of orthodontic treatment because they are flexible. This characteristic of nickel-titanium, which is known for its properties such as high flexibility and elasticity, makes it an ideal choice for initial treatment phases where the teeth are beginning to be guided to their correct positions. Materials like stainless steel, gold, and copper could also be used in orthodontics, but stainless steel is less flexible, gold is corrosion-resistant and copper is known for its ductility and electrical conductivity, hence may not be as ideal for early orthodontic treatment where flexibility is crucial.