Final answer:
The correct statement about radio as an advertising medium is that radio has evolved into primarily a local advertising medium. Option b.
Step-by-step explanation:
The correct statement about radio as an advertising medium is B) Radio has evolved into primarily a local advertising medium. While it's true that radio revenue comes from network advertising, it is not almost 50 percent as stated in option A. Option C is incorrect because radio advertising revenue has not been steadily declining for the last 20 years. Option D is also incorrect as high production costs are not one of the main weaknesses of radio as an advertising medium. Lastly, option E is also incorrect because the high costs of radio advertising do not result in lower reach and frequency.