The fumarate reductase enzyme complex is required for fumarate respiration. This subunit anchors the complex in the membrane and binds a diheme cytochrome b. Binds 2 heme b molecules per subunit, called the proximal (bP) and distal (bD) hemes. Part of an enzyme complex containing three subunits: a flavoprotein (frdA), an iron-sulfur protein (frdB), and diheme cytochrome b (frdC). Belongs to the diheme cytochrome b FrdC family.