Ok. So...
F. is not the answer because their needs to be a change.
G. "This was just one" makes more sense because your sentence will be: This was just one of many such wooden documents found at the site.
H. "Which was one" sounds more like a question to me but if you plug "Which was one" to the sentence it doesn't make sense.
I. "One" makes sense but it feels like it is missing something.
Therefore your answer is G. This was just one
