Answer: Inventory-sales ratio.

**Explanation:** - **Option A (Correct):** The inventory-sales ratio is a lagging indicator because inventories accumulate as sales initially decline and then become depleted as sales recover. This ratio tends to follow the business cycle. - **Option B (Incorrect):** The S&P 500 Stock Index is a leading indicator, as stock prices anticipate economic turning points and provide early signals of economic cycles. - **Option C (Incorrect):** Manufacturers' new orders for consumer goods and materials are leading indicators, as businesses cannot delay meeting demand without ordering, and these orders often reflect changes in business sentiment, which leads the cycle.

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