An antenna is a device that converts
(This question has multiple correct answers)
(A) Electromagnetic energy into radio frequency signals.
(B) Radio frequency signal into electromagnetic energy.
(C) Guided electromagnetic waves into free space electromagnetic waves.
(D) Free space electromagnetic waves into guided electromagnetic waves.

Hint: An antenna is used for converting electrical energy i.e., electric power into radio waves i.e., free space electromagnetic waves and also radio waves into electric power i.e., guided electromagnetic waves.

Complete step by step answer:
-An antenna is a metallic structure that captures or transmits radio electromagnetic waves. Antennas come in different shapes and sizes from little ones that we can see on the top of our roof to watch TV to big ones that capture signals from satellites millions of kilometres away.
-An antenna is a device that converts guided electromagnetic waves into free space electromagnetic waves and vice-versa i.e., it converts electric power into radio waves and vice versa. It acts as a transducer which converts electrical power into electromagnetic waves and also electromagnetic waves into electrical power (Radio waves are part of electromagnetic waves).
-It can be used either as a transmitting antenna in which electrical signals i.e., guided electromagnetic waves are converted into free space electromagnetic waves and radiates them and also used as a receiving antenna which converts electromagnetic waves from the received beam into electrical signals.
Therefore, both options C and D are correct.

Note:The same antenna can be used for both transmission and reception of signals. It has the capability of sending or receiving the electromagnetic waves and it is important in wireless technology.