The question is incomplete, the complete question is shown in the image attached to this answer.
Answer:
a) Br^-
b) FeCl3
c) slower
d) see the first attached image
Explanation:
Aromatic compounds undergo electrophilic substitution sections in the presence of the appropriate electrophile.
In the reaction above, the Br^- nucleophile attacks the Lewis acid FeCl3. Recall that the nitro group is meta directing hence the incoming Br^+ electrophile is directed towards the meta position as shown in the image attached.
Note that the nitro group deactivates the ring towards electrophilic substitution hence the reaction is slower with nitrobenzene than with unsubstituted benzene.
