Answer:
D. "Signals, which travel in straight lines, could not bend around Earth to reach a destination far away."
Step-by-step explanation:
The author here uses the nature of the signal transmission to explain the impossibility of reaching a far destination. A signal (radio signal, television signal, internet signal etc.) can truly only be sent in a straight line. And this can work just fine for some shorter distances; the same city or the same state.
But, imagine a signal that needs to be transmitted from the USA to Europe. The Earth is a globe and due to its curvature and the straight line pattern of signal transmission, it can never bend to reach the receptor on the other side of the planet, which made fast communication in the pre-satelite era quite impossible.