Final answer:
The Nirvana album that broke into the mainstream is Album 2 - "Nevermind," which featured the hit single "Smells Like Teen Spirit" and helped popularize alternative rock.
Step-by-step explanation:
The Nirvana album that broke into the mainstream is b) Album 2 - "Nevermind". Released as their second studio album on September 24, 1991, "Nevermind" catapulted the band into fame, largely due to the success of its first single, "Smells Like Teen Spirit". The album's fusion of pop melodies with punk and metal influences resonated with a wide audience, leading it to become a cultural touchstone of the early '90s and greatly impacting the music industry by bringing alternative rock to a mass audience.