Since other choices are useless in my opinion, it is "exaggeration"
I honestly hope that this helps you.
Answer:
The symbol of commuters as birds illustrates how they come and go without ever experiencing the city.
Explanation:
They go from city to suburb, from the air to it's roost, one could say, but never get to experience what the city has to offer. They don't have the freedom to come and go, because they go there to work. Meaning, having to fulfill some economic needs, responsabilities, etc. They don't go because they feel like going.
They do offer something positive though because, again, they go there to work. A city -a society- needs its body of workers. It needs people to work, doesen't matter if it's from a suburb or not.
The symbolism of the suburbs as a "roost" expresses the safety and comfort of the commuter’s home is wrong because he says:
"The suburb he inhabits has no essential vitality of its own". It is pictured as something sad, empty.
