Explanation

(d)
Write chlorination reaction for all of them to find which gives of the maximum number of monochlorination product.
$(a)\ \ \ CH_{3}CH_{2}CH_{2}CH_{2}CH_{3} + Cl_{2}\overset{\ \ \ \ UV\ \ \ \ }{\rightarrow}$
Cl
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$ClCH_{2} - \left( CH_{2} \right)_{3}CH_{3} + CH_{3} - CH - \left( CH_{2} \right)_{2}$
$CH_{3} + CH_{3} - CH_{2} - CH - CH_{2} - CH_{3}$
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$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ Cl$
$\therefore$ Total 3 monochlorinated products are formed.
(b) $CH_{3} - CH - CH_{2} - CH_{3} + Cl_{2}\overset{\ \ \ \ \ UV\ \ \ }{\rightarrow}$
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$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ CH_{3}$
Cl
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$ClCH_{2} - CH - CH_{2} - CH_{3} + CH_{3} - C - CH_{2}$
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$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ CH_{3}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ CH_{3}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \$
$- CH_{3} + CH_{3} - CH - CH_{2} - CH_{2}Cl$
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$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ CH_{3}$
$\therefore$ Total 3 monochlorinated products are formed.
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ CH_{3}$
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$(c)\ \ CH_{3} - C - H + Cl_{2}\overset{\ \ \ \ \ \ UV\ \ \ \ \ }{\rightarrow}$
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$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ CH_{3}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ CH_{3}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ {CH}_{2}Cl$
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$\ \ \ \ CH_{3} - C - Cl + {CH}_{3} - C - H$
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$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ CH_{3}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ CH_{3}$
$\therefore$ Total 3 monochlorinated products are formed.
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ CH_{3}$
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$(d)CH_{3} - C - {CH}_{3} + Cl_{2}\overset{\ \ \ \ \ \ UV\ \ \ \ \ }{\rightarrow}$
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$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ CH_{3}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ CH_{3}$
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$\ \ \ \ \ \ \ CH_{3} - C - {CH}_{2}Cl$
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$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ CH_{3}$
$\therefore$ Only one monochlorinated products formed.