The question asks which graph represents the behavior of an ideal gas, but the actual graphs for options A, B, C, and D are not visible in the prompt.

For an ideal gas, the governing relation is $PV=nRT$ so the correct graph depends entirely on which quantities are plotted and what is being held constant.

Step 1: Standard graphical behaviors of an ideal gas

Some common correct ideal-gas graphs are:

1. At constant temperature: $P \propto \frac{1}{V}$ So the $P$ vs $V$ graph is a rectangular hyperbola.

2. At constant volume: $P \propto T$ So the $P$ vs $T$ graph is a straight line passing through the origin, provided temperature is in kelvin.

3. At constant pressure: $V \propto T$ So the $V$ vs $T$ graph is a straight line through the origin, again for temperature in kelvin.

4. If temperature is plotted in degree Celsius instead of kelvin, the line does not pass through the origin.

Step 2: Limitation in the given prompt

Since the option graphs are missing, it is impossible to independently derive whether A, B, C, or D is correct from the provided information alone.

Step 3: Comparison with stored answer

The stored correct answer is C. Because the actual graph labeled C is not shown, I cannot verify it from first principles here.

So, strictly speaking, the problem is incomplete in the text provided. However, if I must compare with the stored answer, I can only tentatively accept C based on the supplied key, not on graphical analysis.