The IR spectrum A is the compound heptanal. The structure of the heptanal is 

In the IR spectrum the peaks at below 3000 cm^-1 corresponds to the sp³ carbons i.e. asymmetric and symmetric  -CH₃ stretching. The second peak at 2750 cm^-1is due to fermi resonance of the aldehyde. This is an unique for the aldehyde compounds.

The third peak at  1725 cm^-1is due to aldehyde carbonyl stretching. This confirms  the presence of heptanal.

The IR spectrum B corresponds to the acetic acid (CH₃COOH). The -OH stretching of acid shows an broad peak 3400- 2400 cm^-1. 

The carbonyl stretching of acid carbonyl occurs at  1700 cm^-1. This confirms the structure is acetic acid.

The IR spetrum C corresponds to the 2-butanol because -OH stretching shows a sharp peak at between 3400-3300 cm^-1.  The  peaks at below 3000 cm^-1 corresponds to the sp³ carbons i.e. asymmetric and symmetric  -CH₃ stretching. This confirms the 2-butanol.

 

The IR spetrum D corresponds to the compound 3-pentanone because the peaks at below 3000 cm^-1 corresponds to the sp³ carbons i.e. asymmetric and symmetric  -CH₃ stretching. The characteristic keto carbonyl stretching observed at 1715 cm^-1 . This confirms the presence of 3-pentanone.

Heptanal H