In which instance would it be most to include information not directly related to the main idea

If the options are these:

When the author's purpose is mainly to argue a point
When the author's purpose is mainly to entertain
When the author's purpose is to describe a detailed process
When the author's purpose is to compare two opposing views

I believe the correct answer is C. When the author's purpose is to describe a detailed process.

When describing a detailed process, one needs to include many facts that are not directly related to the question, but still contribute to its illuminating. There is enough room for this information, as it is a detailed description.

When the author wants to argue a point, they should stick to the topic in the narrowest sense. When they want to entertain, they can include whatever information they want, if it causes entertainment. When they want to compare two opposing views, they shouldn't dilute the subject, but should stick to the contrast, the pro and con arguments.