Which property specifies the stacking order of overlapping elements?

The correct choice is z-index, as this property directly influences the stacking order of overlapping elements in the CSS rendering process. When you have multiple elements that overlap each other, the z-index property assigns a stacking order to these elements based on numerical values. Elements with a higher z-index are rendered above those with lower values.

It's important to understand that the default stacking context treats elements without a specified z-index as having a value of auto, which generally places them in the order they appear in the document flow. Only elements positioned using relative, absolute, or fixed positioning can effectively utilize the z-index property.

Other properties mentioned, such as opacity, influence visibility but do not affect the layering of elements. The term stack may relate to the concept of layering but is not a recognized CSS property, while stacking isn't a property either but rather describes the concept of how elements are arranged visually. Thus, z-index is essential for controlling the visual arrangement of elements on a webpage effectively.