Question

calcium ions regulate contraction of smooth muscle cells by binding to
actin.
tropomyosin.
myosin.
calmodulin.
troponin.

Explanation:

In smooth - muscle cells, calcium ions bind to calmodulin. When calcium binds to calmodulin, it activates myosin light - chain kinase, which then phosphorylates myosin and initiates muscle contraction. Actin, tropomyosin, myosin, and troponin are involved in the contraction of skeletal and cardiac muscle in a different regulatory mechanism compared to smooth muscle.

Answer:

calmodulin.