Sample Code:

//Parent Record
Account acc = new Account(Name = 'Test Account');
insert acc;

//Create Related Feed Item Record
FeedItem fi = new FeedItem(ParentId = acc.Id, Body = 'Test Body');
insert fi;

//Create Feed Comment Record
FeedComment fc = new FeedComment(FeedItemId = fi.Id, CommentBody = 'Test Comment');
insert fc;

//Get Feed Comment Record
FeedComment objFC = [Select Id, CommentBody, FeedItemId, ParentId FROM FeedComment LIMIT 1];

//Check Feed Comment Parent Id
System.assertEquals(objFC.ParentId, acc.Id);