_____ than a hundred passengers were killed in the recent train accident.

Passengers are countable nouns, so 'fewer' (not 'less') is the appropriate word. 'No fewer than' is an idiomatic expression used with countable nouns to emphasize a surprisingly large number. 'Less' is used with uncountable nouns. 'Fewest' is a superlative form and does not fit the comparative structure with 'than'. Therefore, 'No fewer' is the correct choice.