Q.Which of the following is optically active ?

Three-Dimensional Representation of Organic Molecules

Because a saturated carbon atom is $sp^3$ hybridised, its four bonds point toward the corners of a regular tetrahedron and lie in three-dimensional space, not flat on the paper. To capture this shape on a two-dimensional page, chemists use the wedge-and-dash (wedge-dash) notation.

• An ordinary (plain) line shows a bond lying in the plane of the paper.
• A solid (bold) wedge shows a bond coming toward the viewer, out of the plane.
• A dashed or hashed wedge shows a bond going behind the plane, away from the viewer.

Using these three kinds of lines, a tetrahedral carbon bearing four groups can be drawn unambiguously. Two drawings represent the same spatial arrangement if one can be turned into the other simply by rotating the whole molecule in space; they represent different arrangements only if no rotation makes them coincide. Since a molecule can be rotated freely, many different-looking wedge-dash pictures are actually identical, while a genuine interchange of two groups (a swap that a rotation cannot undo) gives a truly different spatial arrangement. …