You create a canvas app component. You import the component into a new canvas app and want to add it to the screen. 
Where should you find the component?

Answer options:

A.Under the Screens tab on the Tree view
B.Under the Components tab on the Tree view
C.Under the Popular section on the Insert blade
D.Under the Library components section on the Insert blade
E.Under the Input section on the Insert blade

Correct Answer: D 
The Components are reusable blocks that help you develop the Power Apps applications. Power Apps provides out-of-the-box standard controls (components), like HTML Text, Vertical Gallery, or Edit Form. If your Power Platform solution requires more than standard controls, Power Apps also provides tools to create and reuse the custom code components. 
You don’t need any special software to create a canvas app component. You provision a new canvas app from blank. Then create a component using the controls that Power Apps Studio provides. After you save a component, you can export it in an export file. The best way to maintain and share the component is to use the Component library. You can create a library and import the component. Or when you create a library, the Power Apps Studio creates a new canvas app. You can create or import your component directly in this Library app and then use it for other apps. 
When you need to add a component to the new canvas app, on the bottom of the Insert panel (Number 1), you click on the “Get more components” link (Number 2). The studio opens the “Import components” panel (Number 3) with the Components libraries list. Then you can select the component from the library (Number 4) and push the Import button (Number 5). 

The Power Apps Studio imports the component and puts it under the Library components section on the Insert blade. 

All other options are incorrect. 
For more information about canvas apps reusable components, please visit the below URLs: 

https://docs.microsoft.com/en-us/powerapps/maker/canvas-apps/create-component
https://docs.microsoft.com/en-us/powerapps/maker/canvas-apps/component-library