The correct answer is option 'D' - Kayastha. Al Beruni, an Islamic scholar and historian, wrote extensively about Indian society and culture during the 11th century. In his works, he mentioned several castes, but the Kayastha caste was not included.

Al Beruni primarily focused on the four major varnas or castes mentioned in the ancient Indian scriptures - Brahmana, Kshatriya, Vaishya, and Shudra. These varnas were based on the occupation and social status of individuals.

1. Brahmanas: The Brahmanas were considered the highest caste and were traditionally associated with priests and scholars. They were responsible for performing religious rituals, studying and teaching the Vedas, and guiding the society in matters of spirituality and knowledge.

2. Kshatriyas: The Kshatriyas were the warrior and ruling caste. They held power and authority in society, serving as kings, warriors, and administrators. Their main duty was to protect the society and uphold justice.

3. Vaishyas: The Vaishyas were the merchant and business caste. They engaged in trade, agriculture, and other economic activities. They were responsible for the production and distribution of goods and played a crucial role in the economy.

4. Shudras: The Shudras were the lowest caste in the traditional varna system. They were primarily involved in manual labor and served the other three castes. They were considered to be of lower status and had limited social and economic opportunities.

However, Al Beruni did not mention the Kayastha caste in his works. The Kayasthas are a caste primarily associated with administrative and scribal duties. They were traditionally involved in record-keeping, writing, and serving as scribes to kings and courts. The Kayasthas emerged as a distinct caste in medieval India and are not mentioned in ancient texts.

In conclusion, Al Beruni's writings on Indian society and caste system mentioned the Brahmanas, Kshatriyas, Vaishyas, and Shudras but did not include the Kayastha caste. The Kayasthas emerged as a distinct caste in medieval times and were primarily associated with administrative and scribal duties.