He cut the poster board into 10 equal parts, so you start with none of the poster board being used up.
Since there were 10 sheets of poster board, and now 8/10 of the original amount is left, we know that there must now be 8 sheets left out of the original 10 sheets.
He can make one sign for each remaining sheet, so he can make 8 signs.
The answer is C.
