The answer would most likely be:
The play we are about to read will fill in the details of this story.
*** The choices:
---What is lacking in this description is lacking in our play.
---This prologue does not disclose all the juicy details of the story.
---The play we are about to read will fill in the details of this story.
---A prologue is merely an introduction; it cannot tell the whole story.
If we were to translate this into modern text the line basically says that what was not shown in the prologue will be shown in the play. Among the choices, it is the best translation of what the line meant.