Final answer:
In all three scenarios, the observer is moving towards the source. The rank of the frequencies heard by the observer from lowest to highest is (c), (b), (a).
Step-by-step explanation:
The observed frequency of a sound depends on the relative motion of the source and the observer. In all three scenarios mentioned, the observer is moving towards the source.
(a) The source and the observer are both traveling east. In this case, the observer will hear the highest frequency because the source is moving towards them. So, the frequency heard by the observer in scenario (a) is the highest among the three options.
(b) The source is moving east, but the observer is traveling west. They are moving away from each other. In this case, the frequency heard by the observer will be lower than in scenario (a) but higher than in scenario (c).
(c) The source is moving east, and the observer is traveling west but towards each other. This means they are moving closer to each other. The frequency heard by the observer in this case will be the lowest among the three options.
So, the rank from lowest to highest frequency heard by the observer is: (c), (b), (a).