I found this question in an outside book "not a school book" __ He has filled up the bucket with water. He ........the car. ( a- is washing / b- is going to wash).
My choice is "is going to".
As far as I understand, both the progressive form and be going to are sometimes interchangeable (when they express the future). Yet, the present continuous implies arrangements while the "be going to" form refers to intentions or decisions in addition to "present reality".
That's why I chose "be going to" depending also on this attachment from "A Practical English Grammar".
*Is my choice right?