Lets use process of elimination to find the answer to this question. It's not the first option because a scribes job was mainly in the government district as writers/record keepers meaning they don't really protect the kingdom from invasion. It's not the second options because Merchants were traders, meaning they don't make up the Pharaohs police force because all they do is sell goods to Egyptians. Then it's not fourth option because farmers, servants, and slaves make up way more then 20% of ancient Egypt. Leaving the third option meaning it's your answer.
