$\begin {array} {1 1} (a)\;that\: converts\: electromagnetic\: energy\: into\: radio\: frequency \: signal. \\ (b)\;that \: converts\: frequency\: signal\: into\: electromagnetic\: energy. \\ (c)\;that\: converts\: guided\: electromagnetic\: waves \: into\: free\: space\: electromagnetic\: waves\: and\: vice-versa. \\ (d)\;None\: of \: these \end {array}$
