D. object of a preposition is the correct answer.
A. Flash mobs seem both interesting and safe, unless participants are irresponsible.
The answer would be D) External conflict because its a battle between one character and another so this is just like an ex of Batman vs The Joker, both of them are characters and they are in a fight against each other and this is a external conflict since its a character vs character thing, its not exposition, its not internal conflict because that's a problem in the inside like what should I do, and not resolution because that's like the solution to a problem.