The correct answer is C, Market order. A market order is an order to buy or sell a security immediately at the best available current price, ensuring execution as long as the market is open and there is liquidity.

Step-by-step, the key phrase in the question is “ensure that the trade will be executed today.” Market orders prioritize execution over price, meaning the investor accepts the current market price in exchange for immediate execution. This makes it the most appropriate choice when urgency is the primary concern.

Choice A, a stop order, becomes a market order only after a specified stop price is reached, so execution is not guaranteed if the price is never triggered. Choice B, a limit order, sets a specific price at which the investor is willing to trade, but the order may not be executed if the market does not reach that price. Choice D, a stop limit order, has even more restrictions, as it requires both a trigger price and a limit price, further reducing the likelihood of execution.

Thus, when an investor must sell immediately and guarantee execution, a market order is the correct choice, making Answer C correct.