The correct answer is Sarangi.
The sarangi is a bowed string instrument that is prominently used in many Bihari folk songs. It is a traditional instrument that has been used in India for centuries. The sarangi is made of wood and has a long neck with a gourd body. It has three main strings and several sympathetic strings that vibrate when the main strings are played. The sarangi is played with a bow that is made of horsehair. The sarangi is a very versatile instrument and can be used to play a variety of music, from folk songs to classical music.
The other options are also musical instruments, but they are not as prominently used in Bihari folk songs as the sarangi.
The dholak is a double-headed drum that is used in a variety of Indian music, including folk music. It is made of wood and has two leather heads. The dholak is played with the hands or with sticks.
The harmonium is a keyboard instrument that is used in a variety of Indian music, including folk music. It is a small, portable instrument that is played by blowing into a mouthpiece and pressing down on keys.
The sitar is a long-necked lute that is used in classical Indian music. It is made of wood and has a gourd body. The sitar has seven main strings and several sympathetic strings that vibrate when the main strings are played. The sitar is played with a plectrum.