1. Most likely it is marriage certificate
2. Don't even think of sending original unless INS ask for it specifically. In fact, I will be skeptical even in that case because the way INS works, there is no guarantee that they will send it back.
3. If your marriage certificate was in English, and if English is not the official language in your country, ask one of your friend to translate that in the native language. At the end, translator has to certify under outh that she/he has knowledge of both the languages and above is a true translation (to the best of her ability blah blah...). something like this....
4. No need for notory. If you have a lawyer, let him send it. Lawyer certified document is as good as original.
5. No need to worry. CSC send this RFE to almost everybody.