The collector (1)/ visits the office regularly (2)/ Isn't it? (3)/ No error (4).

Answer

3

Explanation

If the main part of the sentence doesn.t have an Aux. V., the Question Tag uses an appropriate form of do.
Look at the examples given below:
I said that `underset("Aux. V.")underset(darr)(didn.t I) ? `
You recognise me, don.t you?
She eats meat, doesn.t she?
Hence, doesn.t he ? is the right usage.