The trigonometric function that requires a domain restriction of \((0, \pi)\) to be invertible is the sine function, denoted as \( f(x) = \sin(x) \).
The sine function is not one-to-one over its entire domain because it is periodic. Restricting the domain to \((0, \pi)\) ensures that the function is both continuous and monotonic (increasing), which allows it to have an inverse.
Therefore, the correct answer is:
- \( f(x) = \sin(x) \)