What is a key characteristic of layers in ArcGIS Online?

A layer in ArcGIS Online is fundamentally designed to define how geographical data is represented visually on a map. This includes referencing a dataset and detailing the symbols, styles, and text labels that will be used to illustrate that data on the map. By doing so, layers allow users to visualize complex information in a user-friendly manner, making it easier to analyze spatial relationships and patterns.

This characteristic is essential for cartography and data presentation, as it ensures that the information displayed is not only accurate but also understandable and accessible to the end-users. Each layer can represent different types of data, such as points, lines, or polygons, and the styling that accompanies these layers can dramatically affect how the data is interpreted.

Other options lack the essential qualities of layers. For instance, a layer does not function as a collection of web applications or consist solely of geographic coordinates; rather, it is much more comprehensive. Additionally, while data storage is an underlying function of layers, their primary role includes visualization, making the statement that a layer is solely for storage misleading. Thus, the comprehensive role of layers in providing both data reference and visual representation is what makes the selected answer accurate.