Consider an Otto cycle engine with all valve events at TDC or BDC.
A particularly useless/non-extant engine, but I'll go along with the exercise.
On the inlet stroke it inducts one cylinder's worth of volume. On the compression and power strokes nothing goes in or out. On the exhaust stroke one cylinder's worth of volume is expelled.
Is the engine not passing one cylinder's worth of volume in each complete cycle?
If the engine is doing meaningful work or even motoring itself, we are adding fuel and burning it rapidly. This combustion results in end gases different than the intake charge, with different volume, pressure, and temperature. At the extremes, we have:
-zero/negative load resulting in very little flow, volume or mass.
-high load resulting in peak flows, volume or mass.
Because engines have losses, the only way to make your example work is to spin the motor with external power. Even in this case, volume out does not equal volume in, because energy out does not equal energy in. If you could develop a pump to compress a gas 9:1 or so and then decompress it without heating said gas, you'd get to rewrite the Laws of Thermodynamics.
The instances in which I substituted mass flow were because it was a more useful measurement and more relevant to your question IMHO. Engine controls are always designed to reflect this, either by directly measuring mass flow or by measuring volume flow and mass and calculating the resulting mass flow. You say volume flow...volume flow at what location and point in the cycle? It can't be overstated that managing changes in the volume/pressure of a gas at different locations and times in the creation of useful work is the point of the exercise. There are tremendous changes in pressure, temperature, and volume throughout any functioning internal combustion engine including in the intake and exhaust tracts external to the cylinders.
Does that not mean that at constant RPM volume flow is also constant?
No, because the amount of fuel we add has a stronger causal link to load than engine speed, as I explained already. More fuel, more flow. Without fuel, flow might be constant, but intake will not equal exhaust due to heat added in compression.
As altitude increases, if the mixture is not leaned:
A the volume of air entering the carburettor remains constant and the fuel flow decreases
B the volume of air entering the carburettor decreases and the fuel flow decreases
C both the density of air entering the carburettor and the fuel flow decrease
D the volume of air entering the carburettor decreases and the fuel flow increases
The question is inappropriate because the fuel flow term is unique to a given control system and utterly irrelevant in real world operation as every aircraft has a method (automatic, manual, whatever) of managing mixture. It isn't even useful if the mixture control fails...in that case a better indicator of engine health and performance would be provided by EGT.