Which definition best describes a layer in a stack?

• A. A thin sheet of a certain substance
• B. A group of identical items
• C. A substance that is above or under another substance
• D. Different tiers that make something up

Answer: (A)

A layer in a stack is best thought of as a single, flat sheet or level of material within the whole. This matches the common image of a stack as made up of individual sheets laid on top of one another, like a sheet of metal, a slice of bread, or a page of paper. The idea is about one distinct sheet, not a collection of items (which would be a group), not merely the position above or below another substance (which describes relation, not the concept of a level), and not the entire structure made of many levels (which would be the stack as a whole). So the definition that fits is a thin sheet of a certain substance.