Universal Container’s Sales Ops created a report for an external object but she is not able to add a bucket and bucket fields to it. What is the reason for this?

Answer options:

A.Buckets and bucket fields are not available for external objects
B.The custom report type for the report needs to be changed
C.Salesforce reports have a limit on the number of buckets for all reports
D.Bucket fields are not available for standard reports

Correct Answer: A 
Buckets and bucket fields are not available for external objects. 
Option B is incorrect because the custom report type is not related to buckets. 
Option C is incorrect because there is no limit on the number of buckets for all reports. 
Option D is incorrect because bucket fields are available through all reports except external objects. 
Reference: 

https://help.salesforce.com/articleView?id=rd_reports_limits.htm&type=5