Point out the correct sentence -

The correct answer is B. Here "know" takes the wh-word plus infinitive pattern: know how to + base verb. "How to operate" means the method of operating, which is exactly the sense needed.

• A is wrong because "know" does not take a bare "to" infinitive without a question word. "Know to operate" is not idiomatic English.
• C is wrong because "to do operate" puts two verbs in a row. "Do" adds nothing, and "to" must be followed by a single base verb.
• D keeps the right "how to" pattern but repeats the error of C. Remove the extra "do" and you get B.

The core concept is verb complementation: certain verbs take a wh-word plus infinitive (know how to, tell him what to, decide where to). This matters for the English section of Mains too, where precision in verb patterns separates clean writing from awkward writing.

Trap: Aspirants pick D because it looks more complete. In grammar questions, extra words that add no meaning are usually the error.