Q.What would be the shape of the demand curve so that the total revenue curve is

Because $TR = P \times q$, the shape of the demand (price) curve is what forces the $TR$ curve into a given shape.

Total revenue is $TR = P \times q$, so the shape of the demand curve determines the shape of the $TR$ curve.

(a) $TR$ = a positively sloped straight line through the origin. A straight line through the origin has the form $TR = k\,q$, where the slope $k$ is a constant. Since $TR = P \times q$, this means $P = k$ for every output — the price does not change as quantity changes. A constant price at all outputs is a horizontal (perfectly elastic) demand curve, which is exactly the demand curve faced by a firm under perfect competition.

(b) $TR$ = a horizontal line. A horizontal $TR$ curve means total revenue is the same constant at every output: $P \times q = \text{constant}$. Hence $P = \dfrac{\text{constant}}{q}$, which is the equation of a rectangular hyperbola. Along a rectangular-hyperbola demand curve the price elasticity of demand is unity (equal to 1) at every point, so revenue neither rises nor falls as quantity changes.