The details that establish the setting in the passage are:
b. The bustling streets and towering buildings.
c. He left the countryside.
These details indicate the contrasting environments Alex is experiencing: the lively urban landscape opposed to the quiet rural area he came from.
The passage introduces Alex's situation by:
a. It explains his feelings about the new school.
It mentions that his first day was both exciting and nerve-wracking, which reflects his emotional state regarding the new experience.