Which best describes a way in which Postmodernism differs from Modernism?
A. The meaning of the work is left open to interpretation.
B. Style is experimental.
C. Characters are likely to reach an unhappy ending.
D. Themes center around Western culture.

A modernist would tell you the answer is A, a Postmodernist would claim that there is no singularly correct answer.  

Explanation:

Fundamentally, Modernism is marked by the belief that objective truth exists, while Postmodernism posits that truth is subjective and ever in flux.

taking into account that a theme is the central topic a text treats and can be developed through a story, play, poem etc. the following are themes that can be further developed:  

my best friend jeffrey has many unusual interests. (the story could be about each of the unususal interests)

once a man gives his soul over to evil, he can never go back. (this could be a saying or a proverb)

the three sheepdogs lay around the yard all day. (it could be developed detailing how each of these sheepdogs lay around)

a priest's collar has an unusual effect upon people. (it could be developed detailing the different effect upon people)

innocence is a powerful force in a world full of corruption. (it could be developed detailing how is a world full of corruption)

king lear is a play shakespeare wrote later. (it is just a sentence and doesnt give any idea or topic to be discussed in details.

evil often wears the disguise of innocence. (this could be a saying or a proverb)

i guess you are missing the idea that has to be supported, am i right?