Option 1 -> If price is constant regardless of quantity, the demand curve would be horizontal.
Option 2 -> If quantity is constant regardless of price, the demand curve would be vertical.
Option 3 -> When p·q = e (constant), the demand curve forms a rectangular hyperbola showing unitary elastic demand.
Option 4 -> A straight line at 135° would represent linear demand with slope -1.
Hence, Downward sloping rectangular hyperbola -> The equation p_d = e likely represents p·q = e (constant), which is the standard form of a rectangular hyperbola. This curve has unitary price elasticity of demand (elasticity = 1) at all points, meaning percentage change in quantity demanded equals percentage change in price. The curve approaches both axes asymptotically but never touches them, showing an inverse relationship between price and quantity where their product remains constant -> correct