The best statement that describes Simone's error is: Simone did not use the greatest common factor in the equation.
While she factored the expression correctly by using the common factor of 8, the correct greatest common factor (GCF) of 24 and 56 is actually 8, which she used, but her way of expressing the sum inside the parentheses was incorrect for finding a simplified expression. The operations are technically correct; however, she should have structured the expression showing that 24 + 56 equals the same result when the GCF is applied.
If she had done it properly, the expression would retain the overall sum in the factored form with correct results as well. The error stems from not clearly stating or calculating the relationship between the numbers properly.