For good internal control, the billing department should be under the direction of the:

The Credit Manager should be the most important, however, if there is an overall function of "Controller" that has greater authority over all functions, and this is the normal policy in Middle East, then I agree that ...........................A) is the correct answer.

A............................................................................

who ever is controlling,bill is the most important activity it should be done immediately.

A. controller    is the right answer 

An organization can be assured of effective billing if it is placed in the care of the credit manager. Credit managers are fully aware of the financial strengths of customers and are efficient in drawing up bills for various customers alike. Billing is a very sensitive part of the business and just a slight mistake can lead to unexpected deficits.

It should be under dual authority of Controller or Credit Manager (as deemed fit as per organizational structure) along with Sales Manager. As Sales Manager needs to be aware of outstanding balance of different debtors for prospective sales terms but to ensure segregation of duties Controller or Credit Manager should also exercise authority over it.

Sales manager and credit manager .....................................

In my opinion, for good internal control,  controller is responsible. Therefore A.Controller

A controller

<><><><><><><><><>

in my opinion the answer A. controller