The option that is NOT an undesirable effect in the course of signal transmission is "Modulation." Modulation is a necessary process in signal transmission that involves modifying a carrier signal to carry information. It allows the signal to be efficiently transmitted over a medium and extracted at the receiving end. Modulation techniques include amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM).

On the other hand, the other options mentioned—Distortion, Interference, and Noise—are undesirable effects in the course of signal transmission:

1. Distortion: Distortion refers to any alteration or change in the waveform of the signal during transmission, which can result in the loss or corruption of information. Distortion can be caused by various factors such as noise, electrical interference, or imperfect equipment.

2. Interference: Interference occurs when an unwanted signal or noise disrupts the desired signal during transmission. It can be caused by external sources such as other electronic devices, power lines, or atmospheric conditions. Interference can degrade the quality and reliability of the transmitted signal.

3. Noise: Noise refers to any unwanted random signals or disturbances that interfere with the original signal. It can be introduced by various sources, such as thermal noise, electromagnetic interference, or even cosmic background radiation. Noise can reduce the signal-to-noise ratio, making it more challenging to extract the desired information accurately.

Therefore, the correct answer is "Modulation" as it is not an undesirable effect in the course of signal transmission.