Answer:
A. Market Order
Step-by-step explanation:
In a market order, the securities are bought and sold immediately at the current market price prevailing at that time of the day.
Under this, order size is entered such as quantity of stock, the action to be taken i.e buy or sell and no buying/selling rate is mentioned, rather "market" option is checked.
Such market price keeps fluctuating every every moment so the order would be completed at that price which prevailed at that exact moment.
In the given case, the broker upon instructions of the client immediately got the order executed of 500 shares in less than a minute. This is the case of market order wherein the order was executed at current market price.