Question 3
What occupies the empty space above the mercury column in Torricelli's barometer?
The right answer is the one marked above.
A vacuum. That end of the tube is closed, and the mercury did not fall by admitting any air — so no air can have got into that space.
If there were air in it, that air would produce a pressure of its own and push the column below 76 cm. The height would then no longer read atmospheric pressure.