A.
the shortage of rental units is the distance Q3 – Q1.
B.
some renters would be willing to pay a price as high as Rent4 for Q1 units.
C.
no one would have to pay a higher actual price than Rent0, nor would anyone be willing to do so.
D.
there would be a surplus of rental units, but it is impossible to tell how large the surplus is, based on the information provided.