Option D: The set of all real numbers
Step-by-step explanation:
Given that the function
We need to determine the domain of the function.
The domain of the function is the set of all input values for which the function is real and well defined.
In other words, the domain of the function is the set of all independent x - values of the function.
Thus, the function cos (x) is real and well defined in the interval
Hence, the function cos (x) is real and well defined for all real numbers.
Therefore, the domain of the function
is the set of all real numbers.
Hence, Option D is the correct answer.