#### Answer

$y= \cos 2x$

#### Work Step by Step

Upon inspection, we find that the graph shows a cos function with an amplitude of 1 and a period of $\pi$.
In the equation $y=a \cos bx$, $a$ is the amplitude. Since the amplitude is 1, $a$ is equal to 1. To find the value of $b$, we use the formula:
Step 1: Period$=\frac{2\pi}{b}$
Step 2: $\pi=\frac{2\pi}{b}$
Step 3: $b=\frac{2\pi}{\pi}$
Step 4: $b=2$
Substituting the values of $a$ and $b$ in the equation $y=a \cos bx$, we find that the equation of the graph is $y= \cos 2x$.