The correct answer is option (c) i.e. one
The seat belt across your chest should have about one fist width of the slack. Because the seat belt across your chest is required to have minimum fist width of the slack to hold the body in place and to avoid slamming of the body against the seat belt in case of a collision at high speeds. To have one fist width of the slack, makes it a safest to wear it in case of any collision.
