Which of the following instruments is characterized by a fixed bow between the strings?

The instrument characterized by a fixed bow between the strings is the Erhu. The Erhu is a two-stringed instrument, often referred to as a Chinese violin, that utilizes a bow that consists of horsehair and is fixed between the strings. This construction allows the player to create sound by drawing the bow across the strings, giving the Erhu its distinct, expressive timbre.

The design of the Erhu, with its fixed bow, differs significantly from other string instruments, where the bow may be a removable component. This fixed arrangement contributes to the unique playing technique and sound production unique to the Erhu. The musician's fingers press on the strings directly to change pitches, which enhances the connection between the player and the instrument.

The other instruments listed lack this particular feature. The Yangqin is a hammered dulcimer played with mallets rather than a bow, the Didgeridoo is a wind instrument from Australia that produces sound without strings, and the Dizi is a Chinese bamboo flute that involves blowing air and does not utilize a bow at all. Therefore, the Erhu stands out as the correct choice due to its distinctive construction and method of sound production.