Concept:Cost of production is found by adding prime cost and factory overheads, then adjusting for the changes in work-in-progress stock.Explanation:Cost of production = Prime cost + Factory overheads + Opening work-in-progress − Closing work-in-progress.Substitute the given values:4,000+6,000+1,000−2,000=9,000.Sales and returns inwards are not used in this calculation.Answer:N9,000 (Option B).