The derivative function of monotone decreasing function is also monotone decreasing function.
The derivative function of a monotone function is also a monotone function.
None of these are right. Let this monotonic increase/decrease function be a linear function, let it be: y=kx+b, so y'=k, obviously, this is a constant function, not a monotonic increase/decrease function according to the image.
So the fourth answer is the correct answer of elimination.