The correct option is
B few
All the given words are indefinite pronouns. They refer to nouns without saying exactly who or what they are. In the given second sentence, the blank is a pronoun which refers to 'passengers'. Hence, it should be a pronoun referring to countable nouns.
'Many' is used to mean 'many people or things'. 'Few' means 'a small number'. Both these pronouns refer to countable nouns.
The first sentence tells us that many passengers were injured when the train derailed. The group of passengers who escaped unhurt cannot be 'many'. It has to be 'almost none'. Hence, option A is incorrect.
The word, 'few' on a standalone basis (as against 'a few') means 'almost none'. The sentence, 'Obviously, few escaped unhurt' means 'Very few/ almost no one escaped unhurt'. As this sentence falls in line with the meaning of the first given sentence, option B is correct.
Much' means 'a large amount'. It refers to uncountable nouns. As the blank refers to countable nouns, option C is incorrect.
'More' means 'a greater or additional amount or degree' (of something). It is usually used to compare one (or many) noun(s) / pronoun(s) with another. In the given sentences, the passengers who escaped unhurt are not being compared with anything. Hence, option D is incorrect.