Find out which part of a sentence has an error.

Correct option is D

Option (d) contains an error.
The phrase "must waiting" is incorrect. The correct form is "must be waiting". Modal verbs like must are always followed by the base form of the verb. When expressing a continuous action, the correct structure is must + be + present participle (verb+ing).
Grammatical rule used: In English, modal verbs (must, should, can, could, etc.) are followed by the base form of the main verb (also known as the infinitive without "to"). To describe an ongoing action, the structure is modal + be + verb-ing.
Example:
· The students must be waiting for us there.
Explanation of the grammatical rule in Hindi:
अंग्रेज़ी में modal verbs (जैसे must, should, can, could) के बाद हमेशा क्रिया का मूल रूप (base form) आता है। यदि कार्य जारी रहने वाला हो, तो संरचना होती है:
modal + be + verb-ing.
इसलिए, "must waiting" गलत है और सही वाक्यांश होगा "must be waiting"।