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.