Answer
See the figure below.
Work Step by Step
When the box is still inside the capacitor plates, the left side of it must be negatively polarized, and hence the right side must be positively polarized.
Now, if this polarization of the two sides could be frozen, and we can then pull the box out from the capacitor, there will be an electric field inside the capacitor that is created due to this frozen polarization.
Note that the author asked about graphing the electric field inside the box only, not out of it.
The electric field out is more complicated than the inner out.
See the figure below.
