Final answer:
The correct phrase to complete the sentence is "are going to run out," as it conveys imminent certainty, which fits the context provided by the fuel gauge showing that the car will soon run out of petrol.
Step-by-step explanation:
In response to the student's query, the most appropriate phrase to complete the sentence "Look at the fuel gauge. We (will run out / are going to run out / will be running out) of petrol before we reach home" is "are going to run out". This is because the phrase 'are going to' is often used to talk about something that is about to happen in the near future and it implies a certainty based on current evidence – in this case, the fuel gauge reading.
Considering factors like gasoline expansion due to temperature changes, one has to be cautious about interpreting the fuel gauge, particularly in warmer weather when the fuel expands and you actually have less fuel than expected. Furthermore, gasoline is a nonrenewable resource, which adds a layer of importance to understanding fuel usage and gauging requirements accurately, not just to avoid running out before reaching a destination, but also to anticipate fuel scarcity in the future.