how to start

[ostap77]

............of the chief editor being very busy the special correspondent, who had just come back from an overseas assignment,decided to come by his office later.

I have several options for the begining of the sentence. I just don't know which one to choose.

A.Thinking
B.After thinking
C. Having thought

Is this sentence grammatical?

[Alamshar]

None of them fit and it's a bit confusing because you are using different verb forms in the options.

This works:

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.

[ostap77]

None of them fit and it's a bit confusing because you are using different verb forms in the options.

This works:

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.

Could I use " to call in on" here?

[5jj]

Could I use " to call in on" here?

'call/drop in on' the editor;
'drop by (at)/call in at' the editor's office.

[Alamshar]

Could I use " to call in on" hare?

Yes, that's fine