The narrator's attentive concern for fortunato is ironic because
a. they have been friends for a very long time.
b. he is caught up in the spirit of the carnival season.
c. he intends to do him harm.
d. he has been drinking wine.

The concern for fortunato is ironic because Montresor intends to do harm to fortunato. Have in mind that Montressor needs to make sure that his plan goes off without a problem. If Montressor were to act indifferent to Fortunato's health, Fortunato may suspect that Montressor was there not to do good to fortunato. Remember that Irony is a state of affairs, a comment, or an event that is contrary to what you expect.