Answer:
It was a breakthrough in his research.
Step-by-step explanation:
James Giblin's book "The Riddle of the Rosetta Stone" presents the perfect illustrations and descriptions of how ancient hieroglyphs were an important factor. Moreover, the book also presents an interesting overview of ancient Egyptian history for avid historians and even for children interested in archeology.
Based on the given statement, we can know that the "copy of an inscription" that was sent to him by a colleague became the breakthrough in his research. This piece of information helped Jean-Francois Champollion's research and became the breakthrough act that uncovered or revealed everything that's been hidden.
Thus, the correct answer is the first option.