Final answer:
Nickel titanium arch wires are utilized in the early phases of orthodontic treatment because of their superior flexibility, easing the teeth into alignment with gentle forces.
Step-by-step explanation:
Arch wires made of nickel titanium are useful in the early stages of orthodontic treatment because they are highly flexible. This flexibility allows for the gentle application of forces to the teeth, which is ideal during the initial phases when the goal is to correct mild misalignments and to begin the process of moving the teeth. The other materials listed, such as stainless steel and beta titanium, tend to be stiffer and are generally used in later stages of treatment when firmer forces are required to make more significant adjustments to tooth positions. Opti-Flex is not a commonly recognized material in orthodontics and might be a brand name or a product specific to a certain region or manufacturer.