A car travels 42 km on 7 litres of petrol. How far will it travel on 12 litres of petrol?

The car travels $42 \text{ km}$ on $7 \text{ litres}$.

$$\text{Distance per litre} = \frac{42 \text{ km}}{7 \text{ litres}} = 6 \text{ km/litre}$$

$$\text{Distance} = \text{Distance per litre} \times \text{Volume of petrol}$$ $$\text{Distance} = 6 \text{ km/litre} \times 12 \text{ litres} = 72 \text{ km}$$

The car will travel 72 km on 12 litres of petrol.
The correct option is (b).

$\boxed{\text{72 km}}$