The correct answer is C: they show that the worst part of the train ride may be over.
The train is already in northern Chiapas and getting closer to its destination. The readers's attention is being directed at nature and its beauty now, in order to forget the long journey and concentrate on the future ahead. 'Plantain fields soften into cow pastures' is a way to describe peace and tranquility, the word 'soften' is used on purpose here to contrast the harsh train ride. The butterflies are a symbol of awaiting freedom, they 'flutter alongside' in a free and easy style indicating and transmiting the feeling of a new, better beginning. Altogether, the reader gets the feeling that Enrique is entering into a new territory and a new chapter in his life is about to begin.
