The correct answer is C. troponin only.
The smooth muscle cells contain similar elements like the cardiac and skeletal muscle cells including actin, myosin and tropomyosin but they contain troponin.
Smooth muscle tissues are non striated and produce their own connective tissue called endomysium. Smooth muscle cells also lack sarcomeres but have other thin and thick filaments along with actin and myosin contractile proteins.
Due to lack of troponin the cross bridge formation is regulated by the regulatory protein calmodulin instead of the troponin- tropomyosin complex.