Final answer:
The correct interpretation of the mean difference in battery life between brand a and brand b laptops being -3.25 hours is that, on average, brand b's battery life lasts 3.25 hours longer than brand a.
Step-by-step explanation:
The correct interpretation of the mean difference in battery life between brand a and brand b laptops being -3.25 hours is that, on average, brand b's battery life lasts 3.25 hours longer than brand a. So, option 4) On average, brand b's battery life lasts 3.25 hours longer than brand a is the correct interpretation.