The correct factorization gives you B (3x + 5).
The easiest way to find the right answer is to start by looking for a term that will give you the appropriate front term. Since the final, front term is 6x^2. Since we know this is a product of 2x and another, we can divide to find the other.
6x^2/2x = 3x
And we can do the same for the end term using the final term of the polynomial (-15). We know this is a factor of -3 and another.
-15/-3 = 5
Then we put those two answers together to get the final result (3x + 5)