The thin filament is:
a. a bundle of myosin molecules
b. an actin filament
c. an actin filament with its associated troponin complex and tropomyosin
d. a bundle of tropomyosin molecules

The correct answer is option c. "an actin filament with its associated troponin complex and tropomyosin".

Explanation:

Thin filament are a structural component of the muscle comprised mainly of the protein actin, but also it contains the proteins troponin and tropomyosin. The thin filament is approximately 7-9 nm in diameter and are attached to the z discs of the striated muscle. Each component of the thin filament participates in different processes to allow for muscle contraction.