The correct option is
B Although
Option A - 'Still' is not a conjunction, but a conjunctive adverb which means 'regardless'.
For example, 'I have searched the entire house, but I still haven't found my book.' As the given sentence requires a conjunction which means 'despite the fact that Kapil ran fast...', we can't use 'still' here. Thus option A is incorrect.Option C - 'Whether' is a conditional conjunction. We use 'whether' to explain conditions and indirect speech (questions). For example, 'I didn't know whether you'd like it.' As the given sentence doesn't explain any condition we can't use 'whether' here. Thus option C is incorrect.
Option D - 'So' is used to explain reasons. For example, 'I was in the market, so I thought of buying something for you.' As the given sentence doesn't explain reasons, we can't use 'so' here. Thus option D is incorrect.
Option B - 'Although' is a conjunction which explains 'despite something, something happened.' For example, 'Although he was sick, he came to school.' As the given sentence explains that- Despite the fact that Kapil ran fast, he could not catch the train, we can use 'although' here. Thus option B is the correct answer.
'Although Kapil ran fast, he could not catch the train.'