In the text you provided, we know that the resulting tension is growing because of the anger that is being obviously felt by the commuters. Even though what we see is growing in anger, this is not the mood. The mood created for readers because of the apparent growing anger is tension. We know that the answer is not heightened anxiety because there is growth in the events (heightened indicates a static increase). Additionally, we know the answer is not mounting suspense because suspense, by definition, is excitement, which is a good/positive thing, and this is not the case here as the events described here are generally understood to be negative—an undesirable traffic situation.
