Answer:
A. The upward pressure gradient force is balanced by gravity.
Explanation:
A. is correct because the pressure difference is actually generated by gravity. As in the following formula for the pressure at different points:

where
are the pressure at 2 points, ρ is the density of the fluid, g is the gravitational constant, and h is the height difference.
B is incorrect because friction in air is too small to make an effect.
C is incorrect because the Coriolis force is horizontal, not vertical.
D is incorrect because a difference of 500 hPa = 50000 Pa, this is half of the atmospheric pressure.
E is incorrect because temperature cannot generate force.
The pressure at the bottom of the tank is given by:
P = ρgh
P = pressure, ρ = fluid density, g = gravitational acceleration, h = depth
Given values:
ρ = 820kg/m³, g = 9.8m/s², h = 4.5m
Plug in and solve for P:
P = 820(9.8)(4.5)
P = 36000Pa