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# The main purpose of modulation is to

$\begin{array}{1 1}(a)\;\text{Combine two waves of different frequencies}\\(b)\; \text{Aquire changes in carrier wave}\\(c)\;\text{Transmit low frequency signals over long distances efficiently}\\(d)\;\text{Produce side bands}\end{array}$

Audio frequency signal (20Hz to 20KHz) cannot be heard or transmitted over long distances.So modulation using carrier wave is done .
Hence (c) is the correct answer.