Answer: A put option.

**Explanation:** - **Option A (Correct):** A put option allows the holder to sell the underlying asset at a predetermined strike price. If the US equity market declines, the investor can profit by purchasing the asset at the lower market price and selling it at the higher strike price. - **Option B (Incorrect):** A call option allows the holder to buy the underlying asset at a predetermined strike price. This would not be advantageous if the market declines, as the strike price would likely be higher than the market price. - **Option C (Incorrect):** A currency swap involves exchanging payments in different currencies and does not provide a direct mechanism to profit from a decline in the equity market.

Author: LeetQuiz Editorial Team