(Old thread, but someone is bound to Google this in the future)
I went with "below the 9th rib" for the same reason Belleza had above. In UWorld, they say that the pleural space extends inferiorly to the 12th rib in the paravertebral line and that thoracentesis should be performed between the 9th and 11th ribs accordingly.
Similarly, at the mid-axillary line, UWorld says the pleural space extends inferiorly to the 10th rib and that thoracentesis should be performed between the 7th and 9th ribs.
Based on this, the mid-scapular line should probably require thoracentesis between the 8th and 10th ribs because it's roughly mid-way between the paravertebral and mid-axillary lines.
Now the question is whether it should be done above or below the 9th rib. I believe this question came down to whether one considered the fact that you never stick a needle immediately inferior to any rib (unless you're purposely anesthetizing the intercostal nerve). So the answer has to be above the 9th rib. That both fulfills the 8-10th location and doesn't damage the intercostal VAN.
You answer this question, you either had to A) know straight-up the location of thoracentesis at the mid-scapular line or B) you had to have gone through UWorld, knew mid-scapular should have been a rough average of the paravertebral and mid-axillary, then guessed "above" based on the VAN.