To determine how many carbon atoms are in the unknown fuel, we can analyze the balanced chemical reaction:
\[ \text{fuel} + 6 \text{O}_2 \rightarrow 6 \text{CO}_2 + 6 \text{H}_2\text{O} \]
From the products on the right side of the equation, we see there are 6 molecules of carbon dioxide (\( \text{CO}_2 \)). Since each molecule of \(\text{CO}_2\) contains 1 carbon atom, the total number of carbon atoms produced in the reaction is:
\[ 6 \text{ CO}_2 \times 1 \text{ C} = 6 \text{ C atoms} \]
This means that the unknown fuel must contain 6 carbon atoms in order to produce 6 carbon dioxide molecules when it reacts.
Thus, the answer is:
6