Final answer:
The Rickenbacker electric 12-string guitar was popularized by The Beatles and The Byrds in the 1960s.
Step-by-step explanation:
The instrument from the California guitar company Rickenbacker that bands like The Beatles and The Byrds made popular in the mid-'60s is the electric 12-string guitar. This guitar became a symbol of the rock and folk-rock movements of the era. The Rickenbacker 12-string's jangly, rich sound was featured prominently in the music of these bands, influencing the sonic landscape of the 1960s and beyond.