Which order type involves setting a price at which a security must be sold after it is purchased?

The correct answer is a stop order. A stop order is designed to sell a security once it reaches a certain price, effectively creating a trigger point. After a trader purchases a security, they can set a stop order to automatically sell that security if its price drops to or below a specified level. This type of order helps investors manage risk by allowing them to exit their position to prevent further losses.

In contrast, a market order is executed immediately at the best available price and does not involve a specific selling threshold. A limit order specifies a particular price at which the trader wants to buy or sell, but it does not activate if the desired price is not met. A day order is simply an order that remains active only until the end of the trading day unless executed or canceled, and does not address the concept of triggering a sale based on a price movement. Therefore, the nature of a stop order is uniquely suited to the scenario of setting a price at which a security must be sold after purchase.