Final answer:
The I band in the structure of a myofibril contains only thin filaments, which are mainly composed of the protein actin.
Step-by-step explanation:
The I band within a myofibril contains thin filaments only. These thin filaments are primarily composed of the protein actin, with additional components such as tropomyosin and troponin. The I band spans the area between the thick filaments of the A band and is directly adjacent to the Z disc, which serves as the anchoring point for the thin filaments. Thus, the correct answer to the question is B) thin filaments only.