Final answer:
The vihuela, which resembles a small guitar, is the common instrument in mariachi ensembles mentioned in the question.
Step-by-step explanation:
The vihuela, a common instrument in mariachi ensembles, is similar to a guitar.
The vihuela is a key element in mariachi music, often responsible for providing rhythm and harmony. It has a rounded back, five strings, and is slightly larger than a ukulele but smaller than a standard guitar. This instrument, along with the guitarrón, which plays the bass lines, and the trumpet, are essential to the traditional sound of a mariachi band.