The correct answer is D. Catenary.
A catenary is the curve that an idealized hanging chain or cable assumes under its own weight when supported only at its ends. It is the shape of a hanging cable under its own weight, neglecting the effects of air resistance and bending stiffness. The catenary is the curve of equilibrium for a uniform flexible chain or cable under its own weight, and is the shape assumed by a uniform flexible chain or cable when supported only at its ends.
A semicircle is a circle that has been cut in half by a diameter. A triangle is a three-sided polygon. An ellipse is a plane curve surrounding two focal points, such that for all points on the curve, the sum of the two distances to the focal points is a constant.
The catenary is the only one of these shapes that is a curve of equilibrium for a uniform flexible chain or cable under its own weight.