Which of the following triggers muscle contraction

Troponin and Its Role in Muscle ContractionAttached intermittently along the sides of the tropomyosin molecules are additional protein molecules called troponin.These protein molecules are actually complexes of three loosely bound protein subunits, each of which plays a specific role in controlling muscle contraction.One of the subunits (troponin I) has a strong affinity for actin, another (troponin T) for tropomyosin, and a third (troponin C) for calcium ions. This complex is believed to attach the tropomyosin to the actin. The strong affinity of the troponin for calcium ions is believed to initiate the contraction process.Guyton 13e pg: 78