Question

How is written prose more complex than casual speech?

Answer 1

Written prose is more complex than casual speech in that D. it is more structured and organized.
When you are talking to somebody, usually you are not going to pay much attention to the way you organize your thoughts, you just want to get your ideas out there. In prose, however, you cannot do that - or else it will be very difficult to understand your writing. 

Answer 2

D. it is more structured and organized.

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