The correct answer is (x + 5)(x + 4). This can be determined using the method of factoring by grouping.
The general idea is to rewrite the expression as the product of two binomials (in this case, two linear expressions) that add up to the original expression. To do this, we need to find two numbers that multiply to give 20 and add up to 9. These numbers are 4 and 5.
So, we write the expression as:
(x + 5)(x + 4) = x^2 + 9x + 20
Therefore, (x + 5)(x + 4) is the factorized form of x^2 + 9x + 20.