None of them fit and it's a bit confusing because you are using different verb forms in the options.
Thinking that the chief editor was very ........................
You can use "Having thought that" as well but it depends on the context that you are writing it.
Also, I wouldn't use "to come by" here. I would use " to call into". I can't explain why but it doesn't sound right in in the past.
Student or Learner