Explanation:

- G.S. Ghurye (Option 1): He is a prominent Indian sociologist who referred to caste as an "extended kin group". He emphasized the kinship basis of caste, highlighting how jatis often function as large kin networks.

- McKim Marriott (Option 2): He is known for his studies on caste and purity, especially in rural India, but didn't focus mainly on caste as an extended kin group.

- M.N. Srinivas (Option 3): He contributed the concepts of Sanskritization and dominant caste, focusing more on mobility within castes.

- Iravati Karve (Option 4): She studied the kinship systems in India, especially in terms of family and lineage, but is more associated with comparing kinship patterns, not directly with the idea of caste as an extended kin group.

The correct answer is Option 1: G.S. Ghurye.