Final answer:
The actin-myosin II bundles regulated by the troponin complex are found in the contractile ring. The correct option is C.
Step-by-step explanation:
The structure in which actin-myosin II bundles are regulated by the troponin complex is the contractile ring. In muscle cells, troponin, along with tropomyosin, regulates the interaction between actin and myosin II, which is crucial for muscle contraction. The contractile ring, a structure that forms during cytokinesis (cell division), contains actomyosin fibers that are reminiscent of muscle cell fibers but are less organized.
In non-muscle cells such as those containing stress fibers, myosin II does not organize into the highly ordered sarcomeric structures seen in muscle cells, and thus, the troponin complex is not involved in their regulation. In smooth muscle and non-muscle cells like those forming adherens junctions, calmodulin and other proteins such as myosin light chain kinase take over the role of regulating actin-myosin interactions instead of the troponin complex.
The contractile behavior of smooth muscle fibers involves calcium ions binding to calmodulin, which activates myosin light chain kinase to phosphorylate myosin heads, allowing them to interact with actin and cause muscle contraction. The contractile ring during cytokinesis in cell division uses a mechanism similar to that of muscle cells for constriction, which is why it is the correct answer here. Hence, C is the correct option.