A signal emitted by an antenna from a certain point can be received at another point of the surface in the form of
(A) sky wave
(B) ground wave
(C) sea wave
(D) both A and B

Hint: A signal which can be digital as well as analog can travel from one place to another with the help of an antenna. The method of propagation of waves depends upon the type of signal being sent, its frequency and its power. There are a number of factors which determine it.

Complete step by step answer:
An antenna is a device used to transform a signal, traveling on a conductor, into an electromagnetic wave in free space and it can be received at the receiver’s place with the help of a receiver and can be decoded into useful information. Sky wave propagation is the method of propagation of a signal which is reflected or refracted back to the earth from the ionosphere. Antenna can do this work very efficiently.In ground wave propagation the signal travels from one place to another in the area between the surface of the earth and the ionosphere and this happens with the aid of antennas.So, both sky wave and ground wave can be propagated using antennas.
Hence, the correct option is (D).

Note: The atmosphere plays a very important role in transmission of electromagnetic waves. The Ionosphere extends from 80 km to 400km above the surface of the earth. The ionosphere has charged particles which aids in communication.