In the 1920s, the development of practical radio communication was furthered by 1. enthusiasts who built their own sets at home. 2. the theory of modulation. 3. all these answers are correct. 4. both the theory of modulation and the use of vacuum tubes. 5. the use of vacuum tubes.

Hi!

The answer is 3. All these answers are correct.

Practical radio communication was furthered by enthusiasts who were experimenting with Hertz and Maxwell's findings.

By the theory of modulation because it helped establish the earliest method to transmit voice by radio.

By the use of vacuum tubes because they made detection and amplification of radio signals possible. 

So, all of these 3 factors furthered the development of practical radio communication.