Question

troponin:
a has two subunits
b is a long, flexible protein
c has a calcium - binding site
d binds to atp
e is part of the myosin myofilament

Explanation:

Troponin is a complex of three proteins (troponin C, troponin I, troponin T), so it does not have two subunits (A is wrong). Tropomyosin is a long, flexible protein (B is wrong). Troponin C has a calcium - binding site. Myosin binds to ATP (D is wrong). Troponin is part of the thin (actin) myofilament, not the myosin myofilament (E is wrong).

Answer:

C. has a calcium - binding site