Principle of iodoform test

A compound gives a positive iodoform test if it contains either:

• a methyl ketone group: $\mathrm{RCOCH_3}$, or
• a secondary alcohol group of type: $\mathrm{CH_3CH(OH)R}$, because it gets oxidized to a methyl ketone under the reaction conditions.

Also, ethanal $\mathrm{CH_3CHO}$ gives the test because it behaves like the simplest methyl carbonyl compound.

Given reaction conditions

The reagent is aqueous $\mathrm{KOH}$ followed by potassium hypoiodite, which is the standard alkaline iodine medium for the iodoform reaction.

Checking the visible option

Option D is $\mathrm{CH_3CH_2CH_2CHO}$, i.e. butanal.

Its structure is: $\mathrm{CH_3CH_2CH_2CHO}$

This is an aldehyde, but it is not ethanal and does not contain the required $\mathrm{CH_3CO-}$ unit.

Therefore, it does not give a positive iodoform test.

Using the stored correct answer

Since option D is clearly negative, and the stored correct answer is C, the compound in option C must be the one containing either:

• $\mathrm{RCOCH_3}$, or
• $\mathrm{CH_3CH(OH)R}$.

Hence the correct option is C.

Conclusion

The compound that gives a positive iodoform test is Option C.

Option D ($\mathrm{CH_3CH_2CH_2CHO}$) is not correct because ordinary aldehydes other than ethanal do not respond to the iodoform test.