The correct answer is D. They personified abstract concepts
Step-by-step explanation:
In literature, an allegory refers to a literary device in which the writer uses characters, settings, situations, events, etc. to convey a deeper message such as a moral. On the other hand, a morality play was a type of play that emerged in the Middle Ages in which characters usually represent moral attributes in terms of religion.
Morality plays are commonly classified within allegories and especially considered as "naive allegories" because indirectly the characters and situations are used to convey moral and in the case of characters their behaviors, personality and actions personified abstract concepts especially related to good vs. evil. Thus, the attribute that made characters of morality plays reflect that these plays were naive allegories is that thy personified abstract concepts.