Step 1: Understanding the Concept:
For an ideal gas undergoing an adiabatic change, \(PV^\gamma = \text{constant}\). Using \(P = \frac{nRT}{V}\) we get \(TV^{\gamma-1} = \text{constant}\).
Step 2: Compute:
For a rigid diatomic gas, \(\gamma = \frac75\). So
\[ x = \gamma - 1 = \frac75 - 1 = \frac25 = 0.4 \]
Step 3: Why the other options are wrong.
The value 0.25 would result from using \(\gamma = 1.25\). Values 0.30 and 0.60 do not match \(\gamma - 1\) for \(\gamma = 1.4\).
Final Answer:
The value of x is \(0.40\), option (C).
\[ \boxed{0.40} \]