Answer:
From the author's descriptions of the attic door, I get the sense that the writer is extremely anxious about what may be behind the door; thus, I would say that sentences B. or D. would best convey that.
Step-by-step explanation:
The answer you will give, as someone who is more informed on the question than I, wholly depends on the feeling you'd want to evoke.
Sentence A.
This kindles irrepressible curiosity in the reader as to what's in the boxes, not so much anxiety, in my opinion.
Sentence B.
This shows that the room is very old, musty, and has a rather unsettling atmosphere, unperturbed for a very long time. It gives the reader a sense of suspense with the cloud of dust and coughing and gasping for fresh air.
Sentence C.
I feel like this sentence makes the writer feel a little more self-important and may suggest that they are vain, although it is a completely valid point, in my opinion. This supports the eerie ideas surrounding the attic, and what all other people, the author included (before having opened the door), thought was within the room, while also conveying a sense of accomplishment and pride at what they have done.
Sentence D.
This sentence here, I feel, evokes less of a sentiment of anxiety, but more one of curiosity. There are all these boxes around, taped shut, none having been disturbed in the longest time. This makes the reader wonder what can be in those boxes that have been closed and untouched for so long.
Personally, I would choose sentence B. to insert before the sixth sentence. This would make it "Finally, holding my breath, I grabbed the key from my pocket and unlocked the door. As I stepped into the room, a cold breeze brought me chills and a cloud of dust burst into the air as I coughed and gasped for fresh air; I was convinced no one had been in this room for ages, and I was the first explorer. (6) To my surprise, it was a room full of treasure!" (you could mention that, after some looking around, it was concluded that it was a room of treasure, perhaps for some more context).
Awesome tale! I found no errors :)