I think this is another QB
question that has something lost in translation. I guess that "constant
TAS" is in here to eliminate the ram effect on engine performance. If that
is so we are left with thermal and volumetric efficiency, both of which will
affect Specific Fuel Consumption. SFC is, in itself a measure of overall engine
efficiency.
If by "pressure altitude" the questioner means height, the increasing
height will bring reducing temp and therefore better thermal efficiency. True,
the thrust at a given RPM will be falling but the question said "constant
TAS" and if that means constant TAS in level flight you have to increase
RPM at the higher levels to get this. This puts your RPM higher and higher
toward the most efficient range, about 95%N1
So, if this is what the questioner meant, increasing
pressure altitude improves overall engine efficiency and therefore SFC. What
would happen if you increased pressure altitude at constant height by
increasing ambient temperature I wouldn't like to guess...
Advice from experts suggests that this question is not answerable unless you define
the aircraft and engine (high/low by-pass etc). Best advice is to accept the QB
answer and move on.