An order to sell 100 shares of stock at $50, placed when the market price is $45, is known as:

A stop order, also known as a stop-loss order, is designed to trigger a market order once a certain price level is reached. In this case, the order to sell at $50 when the market is at $45 does not fit this description, as it does not activate based on a market price reaching a designated point; it is a direct limitation on the sale price instead.

A market order is executed immediately at the current market price without setting any restrictions on the price. Since the order in question specifies a selling price of $50, it does not qualify as a market order, which would instead execute at the best available price in the market.

A stop-limit order combines features of stop orders and limit orders, whereby a limit order is placed once a specified stop price is reached. In this case, there is no stop price mentioned for the order to sell at $50, making this choice inappropriate for the given scenario.