The correct option is
B Harry was used to getting onto platform nine and three-quarters by now. It was a simple matter of walking straight through the apparently solid barrier dividing platforms nine and ten. The only tricky part was doing this in an unobtrusive way, so as to avoid attracting Muggle attention. They did it in groups today; Harry, Ron, and Hermione went first; they leaned casually against the barrier, chatting unconcernedly, and slide sideways through it and as they did so, platform nine and three-quarters materialized in front of them.
An exposition introduces background information about events, settings and characters in a story.
An exposition helps the reader understand the story better, by describing a person or a scene.
Option B: This paragraph describes a scene where the character Harry, Ron and Hermione walk through platform nine and three-quarters. Hence this is an example of an exposition and option B is correct.
Options A and C: These passages contain significant plot actions 'Frank Bryce...was dead before he hit the floor' and 'Ron floated above the lake bottom...'
Expositions do not reveal plot actions, hence options A and C are wrong.
Option D is wrong because A and C are also wrong.