which gas shows the greatest difference in percent between inhaled and exhaled air?

Answer 1

My suggestion would be CO2
The air we breathe in contains about 0.04% carbon dioxide.
The air we breathe out contains about 4% carbon dioxide.
In other words, exhaled air contains about 100 times the concentration of carbon dioxide that inhaled air does.
There is a corresponding but small change in the O2 content.

Answer 2

Carbon dioxide