Final answer:
The Tanpura is the string instrument used for accompaniment in both Dhrupad and Khayal, providing a continuous harmonic drone as a foundation for the performance.
Step-by-step explanation:
The string instrument that is used for accompaniment in both Dhrupad and Khayal genres of Indian classical music is the Tanpura. This instrument provides a continuous harmonic drone as a foundation to support the melodic structures as the main artist performs, and it is essential in setting the tone and atmosphere for these traditional styles. The sitar, veena, and sarod, while also popular string instruments, serve different purposes within Indian classical music and are generally used as lead instruments rather than for accompaniment.
The Tanpura is a long-necked plucked string instrument commonly used in Indian classical music. It provides a drone-like accompaniment that serves as a reference pitch for the vocalist or instrumentalist.