What is the estimated number of Jewish refugees from arab lands?
a. 100,000- 200,000
b. 300,000- 400,000
c. 500,000- 600,000
d. 800,000- 900,000

A good question, yafa, and some interesting research. Several sources (apparently quoting the same information) mention a figure of around 630,000. As there appears to be an ongoing registration process, I will venture that the number is a bit higher, so I will venture a guess of...

d. 800,000 - 900,000

that is correct