Final answer:
After The Beatles stopped touring, John Lennon pursued a solo music career and became an activist for peace and social justice.
Step-by-step explanation:
When The Beatles stopped touring, John Lennon pursued a solo career in music and became an activist for peace and social justice. He released several solo albums, including the critically acclaimed 'Imagine', and collaborated with other musicians such as Yoko Ono, with whom he formed the Plastic Ono Band. Lennon also became actively involved in political activism and protested against the Vietnam War.