Concept:
NSAIDs are pain and inflammation drugs. Most of them, like ibuprofen and aspirin, carry a carboxylic acid group which makes them acidic.
Step 1:
The oxicams (such as piroxicam and meloxicam) are special because they do not have a carboxylic acid group. Instead they have an enolic acid group, which is what gives them their acidity.
Step 2:
So the feature that sets oxicams apart from the other NSAIDs is the enolic acid group. The other choices (ester, amide) are not the distinguishing acidic group of oxicams.
Answer: Option (2) — Enolic acid group.