Correct option is (c) 49cm
For explanation I would say: A two-by-four wall stud is 3^1⁄2 inches wide, and an interior wall typically has ^1⁄2-inch drywall installed on both sides, bringing the wall thickness to 4^1⁄2 inches. Walls that contain plumbing, such as walls behind sinks, however, should be thicker than standard walls. Two-by-six wall studs are 5^1⁄2 inches wide and they provide a larger stud space for running drainpipes, air ducts or other mechanical elements. With the addition of drywall on both sides, a two-by-six wall is 6^1⁄2 inches wide.