Final answer:
The expression 9x² - 16 can be factored by recognizing it as a difference of squares, (3x)² - 4², which corresponds to option A.
Step-by-step explanation:
To factor the quadratic expression 9x² - 16, we look for two squares that can be subtracted from each other to form a difference of squares. The expression 9x² is the square of 3x since (3x)² = 9x², and the constant 16 is the square of 4 since 4² = 16. Therefore, the expression 9x² - 16 can be rewritten as the difference of squares (3x)² - 4². In this case, the correct form to rewrite the expression for factoring purposes is option A.