I believe the correct answer is B. the use of dialect in speech.
A is incorrect as heroic characters have existed for a very long time (just remember Beowulf); C is incorrect as tragedy was invented back in ancient Greece, which was a looong time before the realist era; D is incorrect because the use of imaginary settings is a characteristic of the romantic era, which preceded the realist era. This leaves us with B, which is evident if you take a look at Huckleberry Finn, for example, which is a good example of Realism.