You create a model-driven app that operates on data from external data sources. 
Please select all correct statements about virtual entities and custom connectors.

Answer options:

A.You can use custom connectors with model-driven apps
B.You can update data in the virtual entity using a registered plug-in
C.You can have a calculated attribute in the virtual entity
D.You map the virtual entities with the external entities using GUID primary key

Correct Answers: B and D 
Dataverse provides virtual entities for integration with external data. A virtual entity is the metadata construct that brings data from other systems into model-driven apps without data storing and replication in the Dataverse. The end-users do not see any difference when they work with virtual entities comparing to the other entities. 
The virtual entity has a virtual data provider and an optional virtual entity data source. Dataverse offers two data providers for virtual entities: OData v4 (the default provider) and Azure cosmos DB. If there is no data provider for your external data source, you can create a custom virtual entity data provider. The custom virtual data providers can support the data CRUD management by using registered plug-ins. You do not need to create a step for each data operation. You need to register a custom data provider using the Dataverse Plug-in Registration Tool that includes the Main Operation Event Handlers (Number 2) within the Data Provider registration screen (Number 1). 

The model-driven app uses only the Dataverse as a data source, including the integration with external data. Developers can embed a canvas app in a model-driven app. Canvas apps can use any of the 300+ connectors that Power Platform provides. Virtual entities (or virtual tables) bring the model-driven apps integration capabilities with external data closer to canvas apps. Option B is correct because you need to register the plug-in for CRUD operations for the virtual entity`s data updates.
Option D is correct because the virtual entity uses the GUID primary key from the external data entities for the Dataverse mapping purposes.
Option A is incorrect because model-driven apps can use only the Dataverse as a data source, including integrating with external data.
Option C is incorrect because the virtual entity cannot have calculated or rollup attributes. These types of operations must be done on the external data side.
For more information about the Dataverse virtual entities, please visit the below URLs: 

https://docs.microsoft.com/en-us/powerapps/developer/data-platform/virtual-entities/get-started-ve
https://docs.microsoft.com/en-us/powerapps/developer/data-platform/virtual-entities/sample-ve-provider-crud-operations