Final answer:
The author ascribes to radio the metaphorical meaning of being a unifying force, as it broadcasts a single message to a vast audience and shapes national identity.
Step-by-step explanation:
In the metaphorical statement "Radio: it ties a million ears to a single mouth," the author attributes to radio the meaning that it serves as a unifying force (option a). This is because radio has the power to broadcast a single message to a vast audience, thus creating a communal experience among listeners, despite their physical separation. The medium of radio shaped American national identity by delivering synchronized content, including news, entertainment, and cultural expressions, to people across the nation. This led to an unprecedented level of shared experiences and the homogenization of regional differences. More than simply a means of conveying information, radio became a tool for societal integration and coherence, and in many ways, it bolstered national unity and reduced linguistic and cultural variances.