Step 1: Formula
Displacement $x = A \cos(\omega t)$.
Acceleration $a = \frac{d^2x}{dt^2} = -\omega^2 A \cos(\omega t)$.
Step 2: Analysis
$a = \omega^2 A \cos(\omega t + \pi)$.
Step 3: Calculation
The phase of $x$ is $\omega t$, and the phase of $a$ is $\omega t + \pi$.
Step 4: Conclusion
The phase difference is $\pi$ rad.
Final Answer: (A)