What configuration allows users to see 2D symbols in the apps?

The choice of web styles is the correct configuration for allowing users to see 2D symbols in the applications. Web styles in ArcGIS Online provide a way to define visual styles that can be applied to layers for map visualization. These styles can include specific symbolizations, colors, and other attributes for display. When web styles are appropriately configured for 2D maps, users can see the intended symbols and designs that have been applied to the features within their maps.

In contrast, default basemaps are pre-defined background layers that set the base context for a map but do not directly influence how the symbols are applied to the data layers. App settings refer to options that govern user interface elements and behaviors but do not specifically address the visual representation of map data. Map viewer options generally focus on interactive features and settings for viewing maps but are not the primary means to configure the display styles of 2D symbols.