Subplots often involve
a. the main series of events
b. positive transitions in character relationships
c. unimportant details that have no bearing on the main plot
d. secondary characters

C. Unimportant details that have no bearing on the main plot

We're looking for things that are similar to protagonist and antagonist. So, we're looking for stuff that is close to it by going through your answers/choices

The main series of events (not your answer)

Positive transition in character relationship (this can be your answer)

unimportant details that uage no bearing on the main point (not your answer)

secondary characters (this can be your answer as well).

Try going with option D. if its wrong try going with B.

If you can choose two then go with D & B .

Good luck!

The error in the sentence is a dangling modifier