Answer
$$y=\frac{\pi}{2}$$
Work Step by Step
$$y=\arccos 0$$
First, we see that the domain of inverse cosine function is $[-1,1]$. $0$ lies in this range, so $\arccos 0$ exists.
Also, it should be noted that the range of inverse cosine function is $[0,\pi]$. In other words, $y\in[0,\pi]$.
We can rewrite $y=\arccos 0$ into $\cos y=0$
In the range $[0,\pi]$, we find that only $\cos\frac{\pi}{2}=0$
That means, the exact value of $y$ here is $$y=\frac{\pi}{2}$$
