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The correct answer is B. Make. So the complete phrase is "Children often make up imaginative stories.
Explanation
"Makeup" is a verb that refers to the action of inventing something; is an action that involves the imagination and creativity to innovate. Additionally, this action is very related to children because in the early years, people tend to ignore reality and create their world. According to the above, the correct answer is B.
I think it would be To Whom It May Concern, because you don't know their relationship status (if that makes any sense)
I could be wrong though...
The answer which is an example of a theme with a dual nature is D. people who work hard learn values but may not prosper.
The other sentences are pretty linear and expected - a person works hard and is successful in life. D is somewhat different, which is why it has two natures, or a dual nature.