Explanation

(a)
The molecule, which is optically active, has chiral centre, is expected to rotate the plane of polarised light.
[image:12_image379.emf]
One chiral center$\Longrightarrow$optically active
[image:12_image380.emf]
Two chiral centres, but plane of symmetry within molecule$\Longrightarrow optically\ inactive$