(C) "In ancient Greek plays, the reading aloud of written texts commonly occurred as part of the on-stage action."
The word "commonly" is what ultimately makes (C) incorrect. Note that this is too large of a claim to be rejected by only the information in the passage. Perhaps it is true that reading aloud commonly occurred on-stage in ancient Greek plays, but this play just happened to depart from what is common. Since this possibility exists, we cannot pick (C) as the best answer.
Note how the language used in (D) differs from that of (C). Instead of claiming that something "commonly occurred," (D) makes the flat-out claim that "people did not read silently." This kind of language can be disproven by one example where people did read silently, and the passage provides such an example.
Does that make sense? Please let us know if you have any other questions!