What represented the first true challenge to the viability of newspapers in the United States?

Answer: Television.

Explanation:

In the 1950s, television became more and more popular, presenting a serious threat to the newspapers. When the first TVs hit the markets, the popularity of the newspapers did not seem to drop. However, this telecommunication medium gradually draw publishers' and editors' attention, and they realized television is a new competitor. Moreover, the publishers were concerned with continued rising costs in their business. Television soon began with on-the-spot coverage of special events, which gradually replaced traditional journalism.