Step 1: Analyze the domain of the function.
For the function to be defined, we must have the following conditions:
- \( 25 - y^2 \geq 0 \) (the square root must be non-negative).
- \( y - 5 \) must lie between -1 and 1 for \( \cos^{-1} \) to be valid.
Step 2: Solve for the domain.
We find that the domain of the function is \( [4, 5] \). Step 3: Conclusion.
The correct answer is (C) \( [4, 5] \).