Carboxylic acid functional group is -COOH

Steps:

• Recall the carboxylic acid functional group: a carbonyl (C=O) bonded to a hydroxyl (-OH) on the same carbon, represented as -COOH.
• Identify circled structures showing only this group, without extra atoms or bonds indicating a larger molecule.
• Eliminate options with modified groups like esters (-COOR) or alcohols (-OH alone).
• Select the option matching isolated -COOH.

Why B is correct:

• It depicts the exact -COOH group, with C double-bonded to O and single-bonded to OH, per the standard organic chemistry definition.

Why the others are wrong:

• A shows an ester (-COOR), where -OH is replaced by -OR.
• C shows a ketone (R-C(=O)-R), lacking the -OH.
• D shows an alcohol (-OH), missing the carbonyl.

Final answer: B