Explanation

(c)
Iodoform test is positive for compounds which have O
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \parallel$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ CH_{3} - C$
group or $2{^\circ}$ alcohol group.
H
|
$(a)\ \ CH_{3} - CH_{2} - C - CH_{3}$
|
OH
has $2{^\circ}$ alcoholic group
O
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \parallel$
$(b)CH_{3} - CH_{2} - CH_{2} - C - CH_{3}$
has$\ CH_{3}CO -$ group
O
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \parallel$
$(d)\ CH_{3} - C - C_{6}H_{5}$
has $CH_{3}CO -$ group
$\therefore$ Compounds in choice (a), (b) and (d) give positive iodoform test.
O
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \parallel$
$CH_{3} - CH_{2} - C - CH_{2} - CH_{3}$
$\because$ This compound doesn’t have $CH_{3}CO -$ or $2{^\circ}$ alcoholic group.
$\therefore$ It does not give positive iodoform test.