Explanation:

When liquid flows out of an orifice at the bottom of a tank, the volume of liquid flowing per second is determined by various factors. However, there are certain factors that do not affect the volume of liquid flowing out of the orifice. Let's analyze each option to understand why.

a) The acceleration due to gravity:
The acceleration due to gravity affects the pressure at the bottom of the tank, which in turn affects the speed at which the liquid flows out of the orifice. According to Bernoulli's principle, the pressure decreases as the speed of fluid increases. So, if the acceleration due to gravity changes, the pressure and speed of the fluid will also change, leading to a different volume of liquid flowing out per second. Therefore, option 'a' is incorrect.

b) The density of the liquid:
The density of the liquid is a measure of how tightly packed its particles are. However, it does not directly affect the volume of liquid flowing out of the orifice. The volume of liquid flowing out per second is determined by the cross-sectional area of the orifice and the speed of the fluid. The density of the liquid only affects its mass, not its volume. Therefore, the volume of liquid flowing out per second remains the same regardless of the density of the liquid. Thus, option 'b' is correct.

c) The height of the liquid above the orifice:
The height of the liquid above the orifice affects the pressure at the bottom of the tank. The greater the height, the greater the pressure and therefore, the greater the speed at which the liquid flows out of the orifice. This means that the volume of liquid flowing out per second will be larger when the height is greater. Therefore, option 'c' is incorrect.

d) The area of the orifice:
The area of the orifice directly affects the volume of liquid flowing out per second. If the orifice has a larger area, more liquid can flow through it in the same amount of time, resulting in a larger volume of liquid flowing out per second. Therefore, option 'd' is incorrect.

In conclusion, the volume of liquid flowing per second out of an orifice at the bottom of a tank does not depend upon the density of the liquid (option 'b'), as it only affects the mass of the liquid and not its volume.