Discussion
Which of the following compounds is infra-red inactive ?
Answer: A
The key's answer is ethylene.
A vibration absorbs infrared radiation only if it changes the molecule's dipole moment. Hydrochloric acid and carbon monoxide are polar diatomic molecules whose single stretching vibration changes the dipole, so both absorb strongly in the IR, and nitric acid is a polar molecule with many IR bands.
Ethylene is a non-polar, centrosymmetric molecule. Its characteristic C=C stretching vibration is symmetric, causes no change in dipole moment, and is therefore IR inactive (it is seen in the Raman spectrum instead). Ethylene does have some other IR-active vibrations, but of the four it is the one whose key vibration is IR inactive, which is the reading the key takes.
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