Correct Answer - Option 3 : limiting power spectral density
SCRAMBLING:
It usually refers to operations carried out in the analog domain and it is accomplished by the addition of components to the original signal or the changing of some important component of the original signal in order to make extraction of the original signal difficult. It is the same as encryption in the digital domain. It is done by a device named Scrambler.
There are two main reasons why scrambling is used:
1. It eliminates the dependence of a signal's power spectrum upon the actual transmitted data, making it more dispersed to meet maximum power spectral density requirements (because if the power is concentrated in a narrow frequency band, it can interfere with adjacent channels due to the intermodulation (also known as cross-modulation) caused by non-linearities of the receiving tract).
2. It also facilitates the work of clock recovery or timing recovery.
In this question options 1, 2, 3 are correct but in the ques, the main function is asked so the most appropriate answer will be an option(3).