19 He has difficulty understanding social .....

Analysis:

The sentence implies a lack of understanding regarding social norms and interactions. 'Etiquette' refers to the customary code of polite behavior in society or among members of a particular profession or group.

Breakdown:

• A etiquette: Correct. Social etiquette refers to the rules of polite behavior in society.
• B behaviour: Incorrect. While behavior is related, 'etiquette' specifically refers to the rules of polite behavior.
• C culture: Incorrect. Culture is broader than social interaction rules.

Translated Question:

19 Ему трудно понимать социальные .....