Explanation

‘Passengers’ is a countable noun, so the comparative must be ‘fewer’, not ‘less’ — ‘less’ is used with uncountable nouns. The idiom ‘no fewer than’ is used to emphasize a surprisingly large number. ‘No less than’ would suit an uncountable quantity, and ‘fewest’ is a superlative that does not fit the construction. Hence ‘No fewer than a hundred passengers’ is correct.