Each of the four drugs listed belongs to a distinct class of lipid-lowering agent with its own separate mechanism: ezetimibe blocks cholesterol absorption at the intestinal brush border, niacin acts through an adipose-tissue receptor to reduce fat breakdown, and colesevelam binds and removes bile acids to indirectly lower cholesterol. Fibrates, the class gemfibrozil belongs to, are the one group of lipid-lowering drugs whose mechanism is built specifically around activating the PPAR-α nuclear receptor in liver and muscle tissue, which switches on genes for fatty acid oxidation and lipoprotein lipase, ultimately lowering triglycerides.
Therefore, the correct answer is Gemfibrozil.