Step 1: Comprehend the Query:
The question requires identifying a characteristic description of Al-Biruni's writing style and content, focusing on his 11th-century work, \textit{Kitab-ul-Hind}, which details his observations in India.
Step 2: Analyze the Options:
Evaluate each option for its association with known travelers:
(1) Caste System Analysis: Al-Biruni's method of explaining the Indian varna system by drawing parallels with ancient Persian social classes for his non-Indian audience is a unique comparative and analytical approach among medieval travelers.
(2) City Descriptions: The portrayal of Delhi and Daulatabad as expansive cities is attributed to Ibn Battuta, a 14th-century Moroccan traveler.
(3) Property Ownership: The assertion that the king owned all land, negating private property among the nobility, is a key observation by François Bernier, a 17th-century French physician.
(4) Flora Descriptions: Detailed and vivid descriptions of the coconut and betel leaf (\textit{paan}), including the former's comparison to a human head, are found in Ibn Battuta's travelogue.
Step 3: Conclude:
The distinct feature of Al-Biruni's account is his explanation of the caste system through Persian analogies.