Final answer:
The domain of the function y=cosθ is all real numbers because cosine is defined for all possible angle inputs and is a periodic function.
Step-by-step explanation:
The question "What is the domain of y=cosθ?" refers to the set of all possible input values (θ) that the function can accept without causing any mathematical complications, such as division by zero or taking the square root of a negative number when dealing only with real numbers.
In the case of the cosine function, y=cosθ is defined for all real number inputs. This is because the cosine function is periodic, repeating every 2π radians, and it is defined for all angles, whether they are positive or negative. Therefore, the domain of y=cosθ is all real numbers.
So, the correct answer to the question is C. All real numbers.