Question

cannot be separated by acid or base extraction choose... choose...

Explanation:

Step1: Recall acid - base extraction principle

Acid - base extraction separates compounds based on their acid - base properties. Compounds with acidic or basic functional groups can be made into salts to change their solubility.

Step2: Analyze first pair

The first pair has a carboxylic acid ($-COOH$) and a ketone. The carboxylic acid can react with a base to form a salt and be extracted into the aqueous layer, while the ketone remains in the organic layer. So they can be separated by acid - base extraction.

Step3: Analyze second pair

The second pair has a methyl - substituted benzene and an aldehyde. Neither of them has a functional group that can be easily protonated or de - protonated in a simple acid - base reaction to change their solubility significantly for separation. So they cannot be separated by acid - base extraction.

Step4: Analyze third pair

The third pair has a methyl - substituted benzene and a phenol. Phenol is acidic and can react with a strong base like $NaOH$ to form a salt and be extracted into the aqueous layer, while the methyl - substituted benzene remains in the organic layer. So they can be separated by acid - base extraction.

Step5: Analyze fourth pair

The fourth pair has a phenol and a carboxylic acid. Both are acidic, but the carboxylic acid is more acidic. They can be separated by using a weak base that will react with the more acidic carboxylic acid first and then a stronger base for the phenol. So they can be separated by acid - base extraction.

Answer:

The pair of a methyl - substituted benzene and an aldehyde cannot be separated by acid - base extraction.