Option D: 8
     1. Let's assume the total amount of petrol in the tank is P liters.
     2. Arun's current petrol consumption rate allows the petrol to last for 10 days. Therefore, his daily petrol consumption is:
        $$\text{Daily Consumption} = \frac{P}{10}$$
     3. If Arun starts using 25% more petrol every day, his new daily consumption will be:
        $$\text{New Daily Consumption} = \frac{P}{10} + 0.25 \times \frac{P}{10} = \frac{P}{10} \times 1.25 = \frac{1.25P}{10} = \frac{P}{8}$$
     4. To find out how many days the petrol will last with the new consumption rate, we divide the total petrol by the new daily consumption:
        $$\text{Number of Days} = \frac{P}{\frac{P}{8}} = 8$$
     Therefore, the tank-full petrol will last for 8 days when Arun uses 25% more petrol every day.