Introduction of a second actor to greek drama created the possibility for a. irony within the chorus
b. dramatic action between characters
c. a protagonist
d. comedy

The answer is B: dramatic action between characters.

Aristotle credits Aeschylus with having introduced a second character on stage, or deuteragonist. This allowed for a dramatic effect since this second character, depending on the play, could switch between supporting or opposing the main character (or protagonist). The deuteragonist would be assigned his or her own plot line, which greatly increased the dramatic possibilities of theater.  

the correct answer would be:  

a. adverb phrase