To convert kilometers to miles, you can use the conversion factor where 1 mile is approximately equal to 1.61 kilometers.
Given that Timmy's van travels an average of 30 kilometers per gallon of fuel, you can convert this to miles using the following formula:
\[ \text{miles} = \frac{\text{kilometers}}{\text{conversion factor}} = \frac{30 \text{ km}}{1.61 \text{ km/mile}} \]
Now, performing the calculation:
\[ \text{miles} \approx \frac{30}{1.61} \approx 18.634 \]
Rounding this to the nearest tenth of a mile gives:
\[ \text{miles} \approx 18.6 \]
Therefore, the closest answer is a) 18.6.