A chiral molecule is one that is not superimposable on its mirror image, typically due to a carbon atom with four different substituents (a chiral center). The molecule that is not chiral lacks such a center or has a plane of symmetry.

Looking at the provided images, the molecule with a plane of symmetry or an internal mirror plane is achiral. The third image depicts 1,3-dichloropropane. Its structure is symmetric, and it has a plane of symmetry, making it achiral and not optically active.

Final Answer: The third molecule (1,3-dichloropropane) is not chiral.