1. The problem states that a retailer buys an item at R480 and applies a 25% cost-based markup. 2. A cost-based markup means the selling price is the cost price plus 25% of the cost price. 3. Calculate 25% of the cost price: $$0.25 \times 480 = 120$$ 4. Add this markup to the original cost price to find the selling price: $$480 + 120 = 600$$ 5. Therefore, the selling price is R600. The correct answer is option a. R600.