A material used to create thin, strong, very lightweight frames, but that results in a material that is not adjustable and is therefore used mainly for frame fronts is:

Carbon fiber is a material known for its high strength-to-weight ratio, making it an excellent choice for creating thin and lightweight frames. Its characteristic properties stem from the way carbon atoms are bonded together, resulting in a structure that is incredibly rigid and strong while still being very light. This rigidity means that once carbon fiber frames are shaped, they do not allow for adjustments; they maintain their formed shape without any flexibility. Because of this lack of adjustability, carbon fiber is primarily used for frame fronts where rigidity and a sleek appearance are desired.

In contrast, other materials like cellulose acetate and nylon often allow for adjustability, as they can be softened with heat and reshaped. Optyl, while also lightweight and strong, allows for some degree of adjustability due to its thermoplastic nature. Thus, the unique combination of strength, lightweight characteristics, and the absence of adjustability firmly places carbon fiber as the correct answer for this specific context in ophthalmic frame design.