The theme is the general message, applicable outside of the work and related to life in general, of a literary work. The plot is what happened in the story, and a symbol is an element of the story that generally works to support an idea of the theme.
I would say the correct answer is A. the theme. Whereas a literary work depicts a plot of some kind, recollecting particular occurrences in the lives of its characters, its theme is a general principle that can be derived from it, a universal message that can also be applied to many other works of art. A single work of art usually has many themes.
The purpose of the bobbin is to hold the thread that is held below the needle. The thread from the bobbin goes up through the plate and connects with the top thread to create a sturdy stitch when the machine is in use.
