You want to optimize the records retrievals from the Dataverse table using Web API Conditional retrievals. 
What should you include in your Web API request to retrieve the record only if it is changed?

Answer options:

A.Prefer
B.ETag
C.odata.track-changes
D.deltatoken
E.If-None-Match

Correct Answers: B and E 
To optimize the records retrieval from the Dataverse when you use the Web API requests, you need to add an If-None-Match header with ETag value. If data has been changed, the response returns “200 OK” HTTP status. If there are no changes, the response returns “304 Not Modified” HTTP status. 
The ETag or entity tag is the tag that OData provides with any retrieved record (Number 1). There are two types of ETags: strong and weak validations. The strong validation checks the resource changes on the binary level. The weak validation checks the resource changes on a semantical level. For the records, OData uses weak validation (Number 2). 

You can use the ETag to query if the record has been changed. For that, you need to add the If-None-Match (Number 1) header with the ETag value (Number 2) to the request. And if there are no changes, you get the “304 Not Modified” status response (Number 3). You can also use another header, “If-Match,” with ETag value for a conditional retrieval. 

All other options are incorrect. 
For more information about the Web API conditional operations, please visit the below URLs: 

https://docs.microsoft.com/en-us/powerapps/developer/data-platform/webapi/perform-conditional-operations-using-web-api#conditional-retrievals
https://docs.microsoft.com/en-us/powerapps/developer/data-platform/webapi/retrieve-entity-using-web-api
https://docs.microsoft.com/en-us/powerapps/developer/data-platform/webapi/perform-conditional-operations-using-web-api