(a)
Number of isomers in hexane are five as follows
$CH_{3} - CH_{2} - CH_{2} - CH_{2} - CH_{2} - CH_{3}$
$CH_{2} - CH - CH_{2} - CH_{2} - CH_{3}$
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$\ \ \ \ \ \ \ \ \ \ \ \ \ CH_{3}$
$CH_{3} - CH_{2} - CH - CH_{2} - CH_{3}$
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$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ CH_{3}$
$\ \ \ \ \ \ \ \ \ \ \ \ CH_{3}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ CH_{3}$
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$CH_{3} - C - CH_{3}\ \ \ \ CH_{3} - \ C - CH_{2} - CH_{3}$
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$\ \ \ \ \ \ \ \ \ \ \ \ CH_{2}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ CH_{3}\ \$
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$\ \ \ \ \ \ \ \ \ \ \ CH_{3}$