Both B and C will give you the correct factorization for this equation.
When you factor a quadratic, you are looking for factors of the last number (-42) that add up to the middle coefficient (1). Below are the factors of -42.
1*-42
-1*42
2*-21
-2*21
3*-14
-3*14
6*-7
-6*7
And since these two are the only ones that add up to 1, we use these in parenthesis, creating the following (x - 6)(x + 7) - which is answer C. You also can switch the order due to the laws of multiplication. If you switch them around you get (x + 7)(x - 6) - which is answer B as well.
