A hollow ball.....
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A sphere and a cylinder can roll straight.
cylinder
Rectangular Hollow Section Pipe means that shape of cross-section of RHS pipe is not round or circle but rectangular or oblong.
A common answer to the riddle "What is round but does not roll?" is a coin. Coins are typically circular in shape but usually remain stationary when placed flat on a surface. Other possible answers could include a plate or a button, as they also have a round shape but do not roll.
A spherical shape like a ball.