a
Here,

\(x=x_{0} \cos (\omega t-\pi / 4)\)

\(\therefore\) Velocity, \(v=\frac{d x}{d t}=-x_{0} \omega \sin \left(\omega t-\frac{\pi}{4}\right)\)

Acceleration,

\(a=\frac{d v}{d t}=-x_{0} \omega^{2} \cos \left(\omega t-\frac{\pi}{4}\right)\)

\(=x_{0} \omega^{2} \cos \left[\pi+\left(\omega t-\frac{\pi}{4}\right)\right]\)

\(=x_{0} \omega^{2} \cos \left(\omega t+\frac{3 \pi}{4}\right)\)              \(...(1)\)      

Acceleration, \(a=A \cos (\omega t+\delta)\)                      \(...(2)\)

Comparing the two equations, we get

\(A=x_{0} \omega^{2}\) and \(\delta=\frac{3 \pi}{4}\)