Answer: c. $19
Step-by-step explanation:
Under the FINRA 5% Policy, a fair and reasonable mark-up or commission is based upon the current market price of the stock not how much the dealer bought it for or rather their cost. As such, when the customer buys, which was the case in this scenario, the mark-up is charged on the inside ask price which in this case is $19.
Were the customer to be selling, any mark-downs will be charged on the inside bid price which in this case is $18.