Option A: Car A
To determine which car maintained the maximum average speed, we need to calculate the distance traveled by each car and then divide it by the number of hours to find the average speed.
     The total payment for each car includes both the hire charge and the cost of diesel. The formula for the total payment is:
     $$\text{Total Payment} = (\text{Distance} \times 6) + \left(\frac{\text{Distance}}{\text{Mileage}} \times 40\right)$$
     Rearranging the formula to solve for distance:
     $$\text{Distance} = \frac{\text{Total Payment}}{6 + \frac{40}{\text{Mileage}}}$$
     Let's calculate the distance for each car:
     ● Car A:  
           ○ Mileage = 8 km/l
           ○ Total Payment = ₹2120
           ○ Distance = $\frac{2120}{6 + \frac{40}{8}} = \frac{2120}{11} = 192.73 \text{ km}$
           ○ Average Speed = $\frac{192.73}{20} = 9.64 \text{ km/h}$
     ● Car B:  
           ○ Mileage = 10 km/l
           ○ Total Payment = ₹1950
           ○ Distance = $\frac{1950}{6 + \frac{40}{10}} = \frac{1950}{10} = 195 \text{ km}$
           ○ Average Speed = $\frac{195}{25} = 7.8 \text{ km/h}$
     ● Car C:  
           ○ Mileage = 9 km/l
           ○ Total Payment = ₹2064
           ○ Distance = $\frac{2064}{6 + \frac{40}{9}} = \frac{2064}{10.44} = 197.70 \text{ km}$
           ○ Average Speed = $\frac{197.70}{24} = 8.24 \text{ km/h}$
     ● Car D:  
           ○ Mileage = 11 km/l
           ○ Total Payment = ₹1812
           ○ Distance = $\frac{1812}{6 + \frac{40}{11}} = \frac{1812}{9.64} = 187.95 \text{ km}$
           ○ Average Speed = $\frac{187.95}{22} = 8.54 \text{ km/h}$
     Comparing the average speeds, Car A has the highest average speed of 9.64 km/h. Therefore, the car that maintained the maximum average speed is Car A.