Final answer:
The Kennedy assassination coincided with The Beatles' first album release in the US, and the contrast between this tragic event and the band's uplifting music helped to boost their popularity.
Step-by-step explanation:
The historic event that coincided with The Beatles' first album release in the US and purportedly boosted their popularity due to their 'breezy' atmosphere was the Kennedy assassination (c). The Beatles offered a middle-class version of the rebellious posturing of the previous generation, soon embodying the essence of suburban youth culture. In a time of turmoil following the Kennedy assassination, their music brought an uplifting refuge for a youth culture traumatized by the political upheaval. Their cheerful sound contrasted with the atmosphere of sorrow and tension, providing listeners with a much-needed escape and thus enhancing their appeal.