Final answer:
The single string instrument commonly used in Samba and Tropicalia music from Brazil is the berimbau.
Step-by-step explanation:
The commonly used single string instrument found in Samba and Tropicália music of Brazil is known as the berimbau. This instrument is pivotal in the performance of Capoeira, a martial art that combines elements of dance, acrobatics, and music, and it is also featured in other Brazilian music genres. The berimbau consists of a single string attached to a wooden bow and a gourd that functions as a resonator. To play the berimbau, the musician strikes the string with a stick while concurrently using a stone or coin to alter the pitch and a caxixi (a type of shaker) to add rhythm.