Step 1: Separate the two properties being compared.
The question asks about heat trapping ability per molecule and atmospheric abundance, which are two different things.
Step 2: Check where CFCs stand on each.
Chlorofluorocarbons are present in only trace, parts per trillion level concentrations in the atmosphere, yet each molecule absorbs infrared radiation very strongly, giving CFCs a global warming potential thousands of times that of carbon dioxide.
Step 3: Compare with the other gases.
Carbon dioxide, methane and nitrous oxide are all present in far larger amounts than CFCs, so they do not fit the low abundance clue.
\[ \boxed{\text{Chlorofluorocarbons}} \]