The correct answer is: D. oscillations.
A control system with excessive noise is likely to suffer from oscillations. This is because noise can cause the system to become unstable, which can lead to the system oscillating between two or more states.
Saturation in amplifying stages is a problem that can occur in control systems when the input signal is too large. This can cause the amplifier to reach its maximum output voltage, which can result in the system becoming unstable.
Loss of gain is a problem that can occur in control systems when the gain of the system is too low. This can cause the system to be less responsive to changes in the input signal, which can make it difficult to control the system.
Vibrations are a problem that can occur in control systems when the system is not properly designed. This can cause the system to oscillate at a high frequency, which can be very annoying and can damage the system.
In conclusion, the correct answer is: D. oscillations.