To find the value of f(25), we need to use the information given.
Given that f'(x) is less than 0 for all x between 1 and 25, this means that the derivative of f(x) is negative over the entire interval (1, 25).
Since the derivative represents the rate of change of a function, a negative derivative means that the function is decreasing throughout the interval (1, 25).
Now, because f is a continuous function with a decreasing rate of change, it must approach its minimum value at the upper bound of its domain, which is 25 in this case.
Since the range of f is (3, 30), and the function is decreasing, the minimum value that f(25) can attain is the lower bound of the range, which is 3.
Therefore, f(25) = 3.
The correct answer is b) 3.