Aspeech given at a graduation ceremony attended by the city mayor should be in style.
a. semi-formal
b. formal
c. informal

It should be formal! (answer b).

The hint in the question is that the city major will attend the ceremony- this tells you that the ceremony will have a more representative, formal character, and will not be an internal celebration among people who know each other (this would make it informal or semi-formal).

C; skin, is the answer!