The correct option is A but
Here, adversative conjunction 'but' is correct because it expresses opposition or contrast between two statements. Here, one of the brothers do not like coffee, but one likes!
'Neither' and 'nor' are alternative conjunctions that present two alternatives, sometimes indicating a choice between them. Example: She didn’t sing, neither did she dance.
'So' is an illative conjunction which expresses an inference. Example: He worked hard, so will succeed!