Explanation

(c)
Iodoform test is given by only those compounds which conatain either
$CH_{3}C = O\ or\ CH_{3}CH - OH$ group
| |
(a) $CH_{3}CH - OH$ (b) $CH_{3}CH - OH$
| |
H $\ \ \ \ \ \ \ \ \ \ CH_{3}$
ethyl alcohol propanol-2
(c) $CH_{3}CH_{2}CH_{2}OH\ \ \$ (c) $CH_{3} - C = O$
Propanol-1 |
H
ethanal
Hence, propanal-1 due to absence of above given groups, does not give positive iodoform test.