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
Knowns
-density decreases
-mixture will enrich
-power will decrease
Unknowns:
-aircraft speed
-propeller control type
-ergo, engine speed and load
-engine VE as a function of speed and load
Potential answers and their problems:
a) Volume flow at the carburetor depends on engine speed and volumetric efficiency. Neither are known. Fuel flow should decrease for a simple carb, as Keith Williams correctly explained. If we assume no changes in engine speed or VE due to the enrichment like someone writing an exam might, this answer could be construed as correct.
b)VE probably goes down as mixture gets progressively richer and load falls. An aircraft with a fixed pitch prop and less power will probably have a slower engine speed too. This is a likely real world answer, but I bet it is wrong on the answer key.
c)The density of air decreases, and as per above the fuel flow likely decreases. I would choose this as the "most correct" answer.
d)This is the least likely answer.
As a test taker I'd choose c), but while cognizant that there is a good chance a) is correct, and with an annoyed knowledge thab b) is almost certainly true, yet "incorrect". My earlier comments about this being an ill-conceived question still stand. Maybe you can tell me which answer is correct.