The correct answer is: To give insight into the relationships among different characters to provide a hint to the audience about what will happen later in the play.
The prologue is an opening segment that gives the audience a short story, connected with the play, before the story itself, it's used to introduce the characters, to give information about their role or to stablish the setting giving background details. It tells the audience what to expect in the play, in this way they won't be confused as the events take place. Euripides was it's inventor, he used it to make the upcoming events comprehensive to the audience.
