Final answer:
The correct second step when solving the equation 4x - 9 = 15 is to subtract 9 from both sides, which would simplify the equation to 4x = 24.
Step-by-step explanation:
The question asks to identify the second step in solving the equation 4x - 9 = 15. To isolate the variable, one should reverse the order of operations, which begins with addressing any addition or subtraction before dealing with multiplication or division. Therefore, the correct answer would be B. Subtract 9 from both sides. This will simplify the equation to 4x = 24, where the next step would then be to divide both sides by 4 to solve for x.