Which of the following is an aldehyde?

Answer: [tex]CH_3CH_2CH_2CHO[/tex] has aldehyde functional group.
Explanation:
Functional groups are specific group of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules.
A. [tex]CH_3CH_2CH_2COCH_3[/tex] has ketone [tex]C=O[/tex] functional group .
B. [tex]CH_3CH_2CH_2COOH[/tex] has carboxylic acid [tex]COOH[/tex] functional group .
C. [tex]CH_3CH_2CH_2CHO[/tex] has aldehyde [tex]H-C=O[/tex] functional group .
D. [tex]CH_3CH_2CH_2COOCH_3[/tex] has ester [tex]RO-C=O[/tex] functional group .
Thus [tex]CH_3CH_2CH_2CHO[/tex] has aldehyde (CHO) group.