I'm pretty sure the answer is the third one!
Answer:
To contradict the message conveyed by the main plot
A subplot can focus on another aspect of the protagonist, or it can follow the story of another main character and his or her development. It can drive home theme and it can aid in world-building. Subplots will enrich your novel and make your story a more compelling read.
D. The social context of the 1960's discouraged innovation and exploration because people were afraid of new technologies and feared being overrun by space aliens.
