What is the equation of the graph below

The equation of the graph below is:

Clearly from the graph that is provided to us we observe that when x=0 the value of the cosine function is: -1

Hence, we put x=0 in each of the given options and check which hold true.

A)

when x=0 we have:

Hence, option: A is incorrect.

B)

when x=0 we have:

Hence, option: B is incorrect.

C)

when x=0 we have:

Hence, option: C is incorrect.

D)

when x=0 we have:

Similarly by the graph of the function we see that it matches the given graph.

          Hence, option: D is correct.