1         a), c), and d); shortwave broadcast uses mostly AM (amplitude modulation).

2         a) and b); the conditions in the upper atmosphere do not affect the short distance propagation of VHF or UHF radio waves. These waves are generally not  reflected in the upper atmosphere, they pass through all its layers and can therefore also be used for satellite communications.

3         c); a repeater station might increase the area a VHF of UHF network can cover, a telecommunication satellite can provide long distance links, and a laptop is useful for some forms of data communication.

4        b); "ROGER" does not mean a name, but stands for the letter "R", in telegraphy this letter means "understood", and to invite the other station to transmit one uses the word "OVER".