Universal Container’s Sales Ops is not able to merge accounts that are associated with Salesforce communities.What is causing this to happen?

Answer options:

A.One of the accounts should not be associated with a salesforce community
B.Accounts that are associated with salesforce communities cannot be merged
C.Manage External User permission should be assigned to the user
D.Manage Community User merge permission should be assigned to the user

Correct Answer: C 
For users, to merge accounts that are associated with salesforce communities, the manage external user permission should be assigned to the user. 
Option A is incorrect because there is no limitation on whether the account should have a salesforce community account associated with it or not. 
Option B is incorrect because accounts that are associated with salesforce communities can be merged 
Option D is incorrect because this permission set doesn’t exist unless it is created and given the same permissions that exist in the manage external user permission. 
Reference: 

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