Step 1: Understanding the adiabatic process.
For an adiabatic process, the relationship between pressure and temperature for a diatomic gas is given by:
\[
P \propto T^{\gamma}
\]
where \( \gamma \) is the adiabatic index, which is \( \gamma = \frac{C_p}{C_v} \), and for a diatomic gas, \( \gamma = 1.4 \).
Step 2: Conclusion.
The value of \( x \) in the equation \( P \propto T^x \) for a diatomic gas undergoing adiabatic change is \( x = 3.0 \), so the correct answer is (A).