The correct option is
D 'show' instead of 'telling'
An exposition is a literary device used to introduce background information or a backstory of a character or plot.Option D: An exposition describes a scene, character or plot and hence reveals more information about it.
Hence, it would be best to 'show' a scene' rather than simply 'telling' what happened.
For example, instead of just saying 'I love chocolate' one can say 'I would smack my lips over any food item with chocolate in it'. Hence option D is correct.
Option A: Dialogue is one of the ways of writing an exposition, but not the only way.
Option B: Using short phrases and sentences isn't a necessity while writing an exposition.
Option C: A single sentence may not always be sufficient to introduce a character or plot. Sometimes it may require a few pages, like in a prologue.
Hence options A, B and C are wrong.