Final answer:
Country music crossed over into the mainstream in the 1970s, aided by artists such as Dolly Parton and John Denver, and by television shows that popularized the genre. The correct answer is option: c) 1970s
Step-by-step explanation:
The 1970s is recognized as the decade when country music crossed over into mainstream popularity. This era brought forth a number of country artists who achieved significant crossover success on the pop charts. Key figures included artists like Dolly Parton, John Denver, and Kenny Rogers, who helped to broaden the genre's appeal beyond its traditional audience.
Their songs featured country elements but were polished with pop production, facilitating their accessibility to a wider range of listeners. Furthermore, the rise of the 'Outlaw Movement' with Willie Nelson and Waylon Jennings also contributed to country music's wider acceptance.
Also of note, during this time, television played a significant role in bringing country music into American living rooms, with shows like 'Hee Haw' and 'The Dolly Parton Show' popularizing the genre.
As for the options provided in the question, option c) 1970s is the correct answer.