Lap 
Distance
(kilometer)

Average speed
(kilometer/hour)

From Graph
Fuel Consumed
(kilometer/liters)

Fuel Consumed
(Liters/kilometer)

P 
15 
15 
60 
0.016 
Q 
75 
45 
90 
0.011 
R 
40 
75 
75 
0.013 
S 
10 
10 
30 
0.033 
$\therefore$ Option B. $Q$ is the correct answer.
Explanation
$P$ covers $15 km$ with an average speed of $15 km/hr.$
From the graph we can see that when speed is $15 km/hr$ then fuel consumed is $60 km/L.$
$\Rightarrow$ For 60 km fuel consumed = $1$ L
$\Rightarrow$ For 1 km fuel consumed = $\frac{1}{60}$ L = $0.016$ L
$\therefore$ Fuel consumed per kilometer for Lap P is 0.016 L/km
$Q$ covers $75 km$ with an average speed of $45 km/hr.$
From the graph we can see that when speed is $45 km/hr$ then fuel consumed is $90 km/L.$
$\Rightarrow$ For 90 km fuel consumed = $1$ L
$\Rightarrow$ For 1 km fuel consumed = $\frac{1}{90}$ L = $0.011$ L
$\therefore$ Fuel consumed per kilometer for Lap Q is 0.011 L/km
$R$ covers $40 km$ with an average speed of $75 km/hr.$
From the graph we can see that when speed is $75 km/hr$ then fuel consumed is $75 km/L.$
$\Rightarrow$ For 75 km fuel consumed = $1$ L
$\Rightarrow$ For 1 km fuel consumed = $\frac{1}{75}$ L = $0.013$ L
$\therefore$ Fuel consumed per kilometer for Lap R is 0.013 L/km
$S$ covers $10 km$ with an average speed of $10 km/hr.$
From the graph we can see that when speed is $10 km/hr$ then fuel consumed is $30 km/L.$
$\Rightarrow$ For 30 km fuel consumed = $1$ L
$\Rightarrow$ For 1 km fuel consumed = $\frac{1}{30}$ L = $0.033$ L
$\therefore$ Fuel consumed per kilometer for Lap S is 0.033 L/km
$\therefore$ We can conclude that the fuel consumed per kilometer was least during the lap Q.
So Option B is the correct answer.