The question's sentence structure employs inversion, common with negative adverbials such as "hardly." This construction establishes a specific link between sentence clauses, following the pattern "Hardly had [action] when [result]," signifying rapid succession of events.
The sentence, "Hardly had the train arrived ________ the passengers rushed to board," aims to illustrate this immediate sequence.
Evaluating the options:
Therefore, 'when' is the correct word to complete the sentence.
Choose the option that best completes the sentence:
"If you want to improve your English, you __________ practice regularly.''
Choose the option that best completes the sentence:
"If I _________ enough money, I would buy a new car.''