ANSWER
d. The set of all real numbers
Step-by-step explanation
The basic cosine function ,
![y = \cos(x)](https://img.qammunity.org/2020/formulas/mathematics/middle-school/3br59xxbn4ebkmj59m4owh6xti43dal15v.png)
is continuous everywhere.
The function is defined for all real values of x.
The domain is the set all values of x that will make the function defined.
The domain is all real numbers.
The correct choice is D.