The correct answer is option 1, Arabic. Al-Birunì`s Kitab-ul-Hind was written in Arabic. Al-Birunì, also known as Abu Raihan Al-Birunì, was a Persian scholar who made significant contributions in the fields of mathematics, astronomy, geography, anthropology, and history. Kitab-ul-Hind, which translates to "Book of India," is one of his most famous works and provides detailed information about the culture, geography, and history of India.

Arabic was the language of science and scholarship during Al-Birunì`s time and was widely used by scholars in the Islamic world. It was common for scholars to write their works in Arabic to reach a wider audience and ensure their ideas would be preserved and widely accessible. Al-Birunì`s choice to write Kitab-ul-Hind in Arabic reflects his intention to share his knowledge with a larger audience and contribute to the body of scholarly works in the Arabic language.

Therefore, the correct language in which Al-Birunì`s Kitab-ul-Hind was written is Arabic.