modulation means...\no to change key.\no to change color.\no to change address.

Answer:

None of the above options are correct. In a general communication - related context, modulation means to change the characteristics (such as amplitude, frequency, or phase) of a carrier wave according to the information - bearing signal.

Brief Explanations:

Modulation in communication systems is about encoding information onto a carrier wave, not about changing key, color, or address as presented in the options. It is a fundamental concept in electronics and communication engineering for transmitting information over long distances.