D.

Oscillator

C.

modulation

Electromagnetic waves are propagated through the following modes:
(i) ground wave propagation
(ii) sky wave propagation, and
(iii) space wave propagation. 

For transmission of signal having frequency range from a few MHz up to 40 MHz, sky wave propagation mode is used. 

Sky wave propagation:

The mode of wave propagation in which the radiowaves emitted from the transmitter antenna reach the receiving antenna after reflection by the ionosphere. 

In amplitude modulation, the amplitude of the carrier wave is varied in accordance with the amplitude of the audio frequency modulating signal.
In this case, the frequency of the amplitude modulated wave remains the same as that of the carrier wave. 




Limitation of amplitude modulation:

1. The amplitude of modulated signal obtaind can be transmitted only by amplifying it first and then fed to an antenna of appropriate size for radiation. 
The size of the antenna matters. 

D.