The difference between the cost price and selling price of an article is

This question tests knowledge of basic retail pricing terms used in commerce.

When a seller buys an article and later sells it for more than it cost, the amount added on top of the cost price to arrive at the selling price is known as mark-up. It represents the profit margin the seller builds into the selling price, and it can be expressed either as a naira amount or as a percentage of the cost price.

The other options describe different pricing concepts. A rebate is a partial refund given to a buyer after a purchase, often as an incentive or adjustment, and is not simply the gap between cost price and selling price. A discount is a reduction made to the normal selling price at the point of sale, for example for bulk buying or prompt payment, rather than the amount added above the cost price. A commission is a payment made to an agent or salesperson for a service, usually a percentage of the value of a sale, and it is unrelated to the difference between what an item cost the seller and what it is sold for.

Whenever a question asks about the gap between cost price and selling price specifically, the correct term is mark-up, not a reduction, refund, or agent's fee.