From the graph we see that tha max=3, min=-1, period is `pi` , and there is no horizontal shift (phase shift.) (We know there is no phase shift since the graph starts at the maximum at x=0 as does the cos function.)
The equation is `y=acos(b(x-h))+k` :
a: a is the amplitude `a=("max"-"min")/2=(3-(-1))/2=2`
positive as the graph is not inverted.
b: `b=(2pi)/p` where p is the period so b=2
h: h=0 as there is no phase shift
k: k is the midline `k=("max"+"min")/2=(3+(-1))/2=1`
The equation is y=2cos(2x)+1.
The answer is (a)