Symmetry and Chirality

Introduction:
Symmetry is an important concept in chemistry, especially when studying the properties of molecules. Chirality, on the other hand, refers to the property of an object or a molecule that is non-superimposable on its mirror image. In this question, we are asked to determine the correct statement regarding elements of symmetry and their effect on chirality.

Axis of Symmetry:
An axis of symmetry in a molecule is an imaginary line around which the molecule can be rotated and still appear the same. The presence of an axis of symmetry can destroy the chirality of a molecule. This is because if a molecule has an axis of symmetry, its mirror image can be obtained by rotating it around the axis. Therefore, the molecule and its mirror image are superimposable, and the molecule is not chiral. Hence, option 'A' is incorrect.

Plane of Symmetry:
A plane of symmetry in a molecule is an imaginary plane that divides the molecule into two halves, such that one half is the mirror image of the other. If a molecule possesses a plane of symmetry, it is superimposable on its mirror image and, therefore, not chiral. Thus, the presence of a plane of symmetry destroys the chirality of a molecule. Hence, option 'B' is correct.

Centre of Symmetry:
A molecule possesses a centre of symmetry if there is an imaginary point in the molecule, such that for any atom on one side of the point, there is an equivalent atom on the other side of the point, in the exact opposite direction. The presence of a centre of symmetry also destroys chirality, as the molecule and its mirror image can be superimposed by inversion through the centre of symmetry. Therefore, option 'C' is correct.

Absence of Plane and Centre of Symmetry:
The absence of both a plane and a centre of symmetry is essential for a molecule to exhibit optical activity. Optical activity refers to the ability of a molecule to rotate the plane of polarized light. In the absence of any elements of symmetry, the molecule is non-superimposable on its mirror image, and its mirror image cannot be obtained by rotation or inversion. This is a characteristic feature of chiral molecules. Hence, option 'D' is correct.

Conclusion:
In conclusion, the correct statement regarding elements of symmetry and their effect on chirality is that a molecule with a plane of symmetry, a centre of symmetry, or both, is not chiral. The absence of both a plane and a centre of symmetry is essential for a molecule to exhibit chirality and optical activity.