Options A and B: When the question requires a choice from a list of items, 'which' is more formal, compared to 'what'.
The
given question asks about the highest mountain in the world, which is one mountain from a choice of mountains. Hence option B is correct and A is incorrect.
Option C: 'Who' addresses a person, and not a thing (mountain).
Option D: 'How' addresses the manner in which something is done, or the state of something. It does not indicate a choice or degree.
Hence options C and D are incorrect.