Q.Identify chiral molecule/s from the following.
a. $CH_3-\underset{OH}{\underset{|}{CH}}-CH_2-CH_3$
b. $CH_3-CH_2-\underset{Br}{\underset{|}{CH}}-CH_2-CH_3$
c. $CH_3-CH_2-CH_2-CH_2Br$
d. $CH_3-\underset{CH_3}{\underset{|}{CH}}-CH_3-CH_3$ (Option d is carried exactly as the book prints it — "CH₃-CH-CH₃-CH₃" with a CH₃ drop-bond — a malformed formula in the original paper.)

A carbon atom bonded to four different groups is called a chiral carbon (marked with an asterisk); a molecule containing one such centre -- for example C-2 of 2-chlorobutane, CH3-*CHCl-CH2CH3, bearing H/Cl/CH3/C2H5 -- cannot be superimposed on its own mirror image, and is called a chiral molecule. This handedness (chirality, from the Greek for 'hand') means a chiral molecule and its mirror image are non-identical stereoisomers of each other, called enantiomers (or enantiomorphs/optical antipodes). To detect this experimentally, plane polarised light (ordinary light passed through a Nicol prism so only oscillations in one plane survive) is passed through the compound: an optically active compound rotates the plane of this light, either dextrorotatory (right, d-/+) or laevorotatory (left, l-/-), by a measurable angle called the optical rotation -- optical activity is the experimentally observed consequence of the underlying structural fact of chirality. Enantiomers share every physical property except the sign of this rotation, and every chemical property except reactivity towards another optically active reagent; an equimolar mixture of both enantiomers, called a racemic modification/mixture ((dl)- or (+/-)-), is optically inactive overall because the equal and opposite rotations of the two enantiomers exactly cancel. A chiral centre's 3-D configuration is drawn on paper either as a Fischer projection (a cross, with horizontal bonds understood to project towards the viewer and vertical bonds away from the viewer) or as a wedge formula (plane-of-paper bonds as plain lines, forward bonds as solid wedges, backward bonds as broken wedges). This chirality concept is the direct prerequisite for understanding why SN1 substitution racemises a chiral substrate while SN2 inverts its configuration.