What must be done to use a partial view inside another view's rendered output?

To use a partial view inside another view's rendered output, the correct course of action is to understand that partial views can be utilized seamlessly without any special registration or configuration. When a partial view is called from within another view, it is inherently recognized by the rendering context of that view, as long as the partial view file is placed in the correct folder and follows the naming conventions.

When you include a partial view within a parent view, you typically use Razor syntax such as @Html.Partial("PartialViewName") or @Html.RenderPartial("PartialViewName"). This approach directly integrates the contents of the partial view into the parent view's output, making it straightforward to modularize your UI and maintain clean code.

The other options hint at misunderstandings about how partial views work within the ASP.NET MVC framework. For instance, registering a view in Startup.cs would be unnecessary for simply rendering a partial view, as view discovery happens automatically based on the folder structure and naming conventions in the MVC application. Thus, those details underline why the notion that no additional requirements exist when using a partial view within another view is indeed accurate.