The derivative function of monotonically increasing function is also monotonically increasing function.

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.