Radial symmetry is symmetry where, no matter what line you cut across the shape you are bisecting (so long as that line goes through the center of the shape), you are left with two similar halves. The only organism on this list which can be divided this way is the starfish, which is your answer. Hope that helped! =)
Radial symmetry depicts a body plan of an organism, which can be divided into two equal halves when a plane from any angle passes through central axis of the body.
Starfish is radially symmetrical organism as any plane can divide its body into two equal halves.
On the contrary, flatworm, butterfly, and tadpole show bilateral symmetry ( only single axis/ plane can divide the body into two equal halves.
