Answer:
The crop that transformed the British West Indies from a society of small landowners using indentured servants to plantation owners using slave labor was sugar. Some key details:
• Prior to the development of large-scale sugar production in the 1600s and 1700s, the economies of the British West Indies relied mainly on products like tobacco, cotton and indigo. Small landholders used indentured servants, mostly poor Europeans, as laborers.
• The introduction of sugar cane from eastern Mediterranean colonies radically changed the economic system. Sugar was extremely profitable and demand was high in Europe. It required vast tracts of land and intense, year-round labor to produce.
• Planters switched to purchasing African slaves, who were seen as better suited to the difficult conditions of sugar cultivation. The transatlantic slave trade expanded massively to supply labor for the sugar plantations.
• The rapid growth of the sugar industry concentrated land ownership in the hands of a small elite who could afford large numbers of slaves. The system of slave plantation agriculture replaced the old pattern of small tobacco holdings using indentured labor.
• By the late 1700s, sugar dominated the economies of British Caribbean colonies and the region had become heavily dependent on the slave labor of imported Africans to produce sugar for export.
So in summary, the introduction and spread of sugar cane and the attendant rise of sugar plantation agriculture was the primary factor that transformed the labor system, land distribution and overall society of the British West Indies - transitioning it from a region of small landholders utilizing indentured servants to plantation owners reliant on African slaves.