Use suitable auxiliary verb to fill in the blank space: You _________ catch the train at once.

Correct answer: must

Explanation: The blank requires an auxiliary verb that conveys the appropriate sense of obligation, possibility, or intention for the sentence 'You _________ catch the train at once,' which implies urgency or necessity. 'Must' is the correct auxiliary verb here, as it indicates a strong obligation or necessity to act immediately, fitting the context of catching the train without delay. will ❌: Suggests future intention or prediction, not urgency. can ❌: Indicates ability, not obligation to catch the train. shall ❌: Suggests future obligation or intention, less urgent than 'must.' must ✅: Expresses strong necessity or obligation, perfect for immediate action. Correct Answer: D. must