Explanation

(a)
Iodoform test is given by the compounds containing either
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$CH_{3}CO -$ roup or $CH_{3}CHOH$ group.
The structures of the given compounds are as
$CH_{3}CH_{2}CH_{2}CH_{2}OH$
$CH_{3}COC_{6}H_{5}$
$CH_{3}CHO$
$CH_{3}COC_{2}H_{5}$
$\therefore\ n$ butyl alcohol does not give iodoform test because it does not possess the
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$CH_{3}CO -$ or $CH_{3}CHOH$ group.