Step 1: Start by separating the reasons a limb is removed into two groups. Absolute reasons include gas gangrene, diabetic gangrene and an irreversible loss of blood flow, while relative reasons include injury, tumours and severe loss of function.
Step 2: When all causes are tallied, trauma to the limb stands out as the leading reason for amputation, since severe injury can wreck the vasculature or shatter the limb beyond salvage.
Step 3: The dominant source of such serious limb trauma in India is road traffic accidents, reflecting the high burden of road injuries. The gangrene options and tumours, though valid indications, account for fewer cases, so the road traffic accident option is correct.
\[\boxed{\text{Road traffic accident}}\]