The statement which accurately describes a cause-and-effect relationship described in this excerpt is:
C. Because the French soldiers are tearing down a ruined wall in a fort, they discover the Rosetta Stone.
A relationship of cause and effect implies that we have two events. One event is the consequence of the other or, if we invert the reasoning, one event causes the other.
In the paragraph, we do not know the relationship between Napoleon's departure and the French army's decision to stay. The author does not make it clear.
However, the relationship between the discovery of the stone and the wall in a fort is made clear in the following excerpt:
". . . the stone had been unearthed in an old fort neat the town of Rosetta. . . French soldiers were tearing down a ruined wall in the fort when they came upon the slab."
Thus, the relationship is clear: 1. the tearing down of the wall. 2. coming across the stone.
In conclusion, finding the stone is the consequence of tearing down the wall. Tearing down the wall is the cause of finding the stone. Letter C is the correct option.