Final answer:
The domain of the sine function is all real numbers, as the sine function can accept any real-number angle as its input.
Step-by-step explanation:
The domain of the sine function refers to the set of all possible input values (angles) for which the sine function is defined. According to trigonometry, the sine function can accept any real number as its input since it represents the ratio of the length of the opposite side to the length of the hypotenuse in a right-angled triangle as the angle varies. Thus, given the periodic nature of the sine function, it oscillates between +1 and -1, but the angles (i.e., the domain) can be any real number. Therefore, the correct answer to the question is a) All real numbers.