Concept:The correct choice depends on collocation: "sell-by" is used before "date" to show the last day a product should be sold.
Explanation:The blank requires a word group that correctly describes a "date".
The standard English expression is "sell-by date".
It means the final date on which a shop should offer a product for sale.
For example, perishable food items carry a sell-by date to guide stores and consumers.
Option B, "sell-out", means an event has no tickets left or the act of selling all items.
Option C, "sell-off", means a quick sale of assets, usually at low prices.
Option D, "sell-on", is not a standard phrase in this context.
Therefore, option A completes the sentence correctly.
Answer:A. sell-by