The correct answer is: D. Pulse broadening.
Disturbance along the fiber such as vibrations, discontinuities, connectors, splices, source/detectors coupling result in pulse broadening. This is because the light waves traveling through the fiber are affected by these disturbances, which causes them to spread out over time. This can lead to signal degradation and loss of information.
Modal noise is caused by the different modes of light traveling through the fiber. These modes are caused by the different ways in which the light can travel through the fiber, and they can interfere with each other, causing noise.
Inter-symbol interference is caused by the overlap of two or more pulses. This can happen when the pulses are too close together, or when the fiber is not properly terminated.
Infrared interference is caused by the absorption of infrared light by the fiber. This can cause signal degradation and loss of information.
In conclusion, the correct answer is: D. Pulse broadening.