The invigilator _____ to know how long the examination _____ going on
Answer Details
The best option to complete the gap is "wanted to know how long the examination had been going on."
The word "wanted" indicates that the invigilator had a desire to know something, which in this case is the duration of the examination. The use of "had been" indicates that the examination had started before the invigilator's inquiry, so it had already been ongoing for some time.
Therefore, the correct sentence would be: "The invigilator wanted to know how long the examination had been going on."