What was the primary socio-economic factor behind the proliferation of new sub-castes (Jatis), particularly the Kayasthas, during the early medieval period in India?

Options:

• A: The decline of the Buddhist monastic order which forced monks to rejoin lay society.
• B: The massive influx of foreign invaders who were assimilated into the Vaishya varna.
• C: The expansion of maritime trade which created specialized merchant guilds.
• D: The expansion of state bureaucracy and the need for maintaining complex land grant records.

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