Answer:The sentence that has a more formal style and tone is: the benin plaques are important for many reasons, but namely because they document two prominent points of euro-african contact, one of which was peaceful and the other not.
Formal style implies getting rid of informal language and language that is usually used when speaking. The other options given contain informal phrases which are usually found in spoken discourse like "happen to document", "tons of reasons" and "are on the map".
Step-by-step explanation: