Al-Biruni's "Kitab-ul-Hind" is written in Arabic because Arabic was a scholarly and scientific language widely used by scholars and intellectuals during the Islamic Golden Age, especially for works related to various fields of knowledge, including philosophy, science, and history. Al-Biruni, being a Persian polymath and scholar, wrote many of his works, including "Kitab-ul-Hind," in Arabic to ensure their accessibility and influence in the broader Islamic world. Arabic served as a common language of communication and dissemination of knowledge in the medieval Islamic world, making it the medium of choice for scholarly writings.

So, the correct option is [2].