Friends turning against each other is the clearest example of a theme. Option A is correct.
The theme in a story is its underlying message, or 'big idea' that the author aims to convey. To put it simply, it is the critical belief about life that the author is seeking to express in the writing of a novel, play, short story or poem. These ideas, beliefs tend to be universal in nature so that they invoke human experience, regardless of race or language.
