Final answer:
The major reason for the decline in the viewing audiences of the four major networks is the growth of cable television.
Step-by-step explanation:
The major reason for the decline in the viewing audiences of the four major networks is the growth of cable television. Cable television, which emerged in the 1980s and expanded in the 2000s, provided viewers with more options and specialized channels that appealed to specific groups of consumers.
This resulted in viewers being able to watch nearly anything they wanted, bypass commercials, and record programs of interest. The availability of cable television and the proliferation of channels led to a decline in the audience for the major networks.