What is the definition of the word theme?

a. a major message or lesson developed in a literary work
b. the emotional atmosphere of a literary work
c. an important topic developed in a literary work
d. the series of events that happen in a literary work

c. an important topic developed in a literary work

a. a major message or lesson developed in a literary work Could this be the answer?

Yes, you are correct. The definition of the word "theme" can also be described as a major message or lesson developed in a literary work.