Interpersonal skill requires that

Interpersonal skill is about how well you get your message across to another person and how well you understand theirs. What matters is effective communication, not the number of languages you know. So option D is correct.

• A is wrong. Knowing three languages is a language-proficiency point, not an interpersonal one. The definition does not ask for any particular trilingual combination.
• B is wrong for the same reason. Fixing English and Hindi turns it into a bilingual-policy point and still ignores the actual skill.
• C is wrong because "good knowledge of any language" is vague and misses the point. Knowing a language is not the same as communicating effectively in it with the listener.
• D is correct. Effective communication, adjusted to the audience and the language they understand, is the core of interpersonal skill. It is why the topic sits under soft skills and civil service conduct.

Core concept: interpersonal skill rests on communication, empathy and feedback. For Mains, this links to the ethics paper, where a civil servant must communicate with citizens in a language they actually understand.

Trap: Students pick A or B because a multilingual option looks more impressive. The question asks about the skill, not the qualification.