Explanation

(b)
There are four structural isomers are possible for $C_{4}H_{9}Cl$
(a)$CH_{3}CH_{2}CH_{2}{CH}_{2}Cl$
(b)$CH_{3} - CH_{2} - CH - CH_{3}$
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$Cl$
(c)$CH_{3} - CH - CH_{2}Cl$
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$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ CH_{3}$
Cl
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(d)$CH_{3} - C - CH_{3}$
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$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ CH_{3}$