Correct answer: Troponin and myosin - Complex proteins in striated muscles
Troponin and myosin are both protein components of striated muscle involved in contraction. The other pairs mix incorrect categories: calcitonin is not a thyroid hormone pair with thyroxin in the intended sense, pepsin is a stomach enzyme but prolactin is a pituitary hormone, and secretin is a peptide hormone while rhodopsin is a visual pigment.