The correct option is Mechanised operations over large farms.
Explanation
Commercial grain farming is a form of extensive agriculture practiced primarily in the mid-latitude grasslands of the world, such as the Prairies of North America, Pampas of Argentina, Steppes of Eurasia, and Downs of Australia. It is oriented exclusively towards the market.
Analysis of Options:
- Mechanised operations over large farms is Correct: The defining characteristics of commercial grain farming are large landholdings (often hundreds of hectares) and a high degree of mechanisation. All farming operations, from ploughing to harvesting, are fully mechanised to manage extensive areas efficiently with low labour input.
- Rainforest cultivation is Incorrect: Rainforest cultivation is typically associated with shifting cultivation (slash and burn) or specific plantation crops. Commercial grain farming requires the open, semi-arid to sub-humid environments of temperate grasslands.
- Intensive labour use is Incorrect: This farming system is capital-intensive, not labour-intensive. Due to heavy reliance on machinery, the labour input per unit of land is very low compared to intensive subsistence farming.
- Small landholdings is Incorrect: Small landholdings are characteristic of intensive subsistence agriculture (common in densely populated regions like Monsoon Asia). Commercial grain farming relies on economies of scale derived from very large farms.
Key Takeaway: Commercial grain farming is characterized by extensive land use, monoculture (mainly wheat), high capital investment, and heavy mechanisation, contrasting sharply with intensive subsistence farming.