To sell a box spread, an investor must sell a European call option and buy a European put option with a specific strike price (X1), and simultaneously buy a European call option and sell a European put option with a higher strike price (X2). This strategy is known as a short box spread strategy. A box spread is a complex strategy that involves four options with the same expiration date but different strike prices. The goal of this strategy is to create a risk-free position that can potentially generate arbitrage profits. When the market value of a box spread is higher than the present value of its payoff, it indicates that the options are overpriced. By selling the box spread, the investor can lock in a guaranteed profit regardless of the future price movements of the underlying asset. This is because the payoff from the box spread at expiration will always be equal to the difference between the two strike prices, regardless of the price of the underlying asset.

Choice A is incorrect. This choice suggests buying both a call and a put option at strike price X1, and selling both a call and a put option at strike price X2. This strategy would actually create a long box spread position, not the short box spread position that Harris needs to profit from an inflated market value of the box spread.

Choice B is incorrect. Selling both a call and put option at strike price X1, while buying both at strike price X2 would also result in creating a long box spread position. Again, this is contrary to what Harris needs to do in order to exploit the arbitrage opportunity.

Choice C is incorrect. This choice suggests buying a call and selling a put at strike price X1, while selling another call and buying another put at higher strike price X2. However, this combination does not form any type of box spread strategy (neither long nor short), hence it cannot be used by Harris for his purpose.