The correct answer is: A. T/409.
In MACLS system, if approach signal becomes blank, T/409 authority is issued from the station in rear. This is because T/409 is a type of authority that allows the aircraft to continue its approach even if the approach signal is not available. This type of authority is only issued by the station in rear, which is the station that is located behind the aircraft.
The other options are incorrect because they do not allow the aircraft to continue its approach if the approach signal is not available. Option B, T/408, is a type of authority that allows the aircraft to enter the holding pattern. Option C, T/407, is a type of authority that allows the aircraft to land on a runway that is not aligned with the approach path. Option D, T/406, is a type of authority that allows the aircraft to land on a runway that is not equipped with an instrument landing system.