Frank was about to leave when

[ShanQ]

Original text:
Frank was about to leave when he noticed a large packing case lying on the floor.

MCQ
Frank was ____ leaving when …
(a) almost (b) just (c) only (d) hardly

The textbook answer is (b).
But I feel (a), (b) and (d) are all acceptable.

The original sentence "was about to..." means only a very short moment before Frank left, he noticed a large packing case lying on the floor.

If I use "hardly … when …", I would write the sentence like this:
Frank had hardly left when he noticed a large packing case lying on the floor.

I guess the sentence is also acceptable：
Frank was hardly leaving when he noticed a large packing case lying on the floor.

Thank you so much for your help.

 

[grassy]

But I feel (a), (b) and (d) are all acceptable.

I disagree. Almost and hardly tell us whether he was leaving or not. They are not temporal.