You should provide an affidavit with the details of your service, such as dates of service and what was your rank and assignment and the manner of your discharge. If your answers to #15A and #18 are "yes",you may need to resubmit an I-485 answering those questions correctly and another affidavit stating that you didn't understand the questions the first time. You might want to retain an immigration attorney to help you with this.
Here's the definition of an affidavit from USCIS:
A document in which a person states facts, swearing that the facts are true and
accurate. The person (the affiant) must include their full printed name and address, date and place of birth, relationship to the parties, if any, and complete details concerning how the affiant acquired knowledge of the events. The affiant should sign the affidavit under oath and the signature should be witnessed by an official, such as a notary public.
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When you indicated you were in the military and answered "no" to questions relating to military service, that created a conflict and USCIS wants an explanation of some kind. If the RFE is requesting a sworn statement regarding your military service and specifically mentions the questions you referenced, then you need to explain why you answered "no" to those questions. In part C, if you did not explain it was mandatory military service, you need to explain that. If you believe you were correct in answering "no" to the other two questions, explain why. If you now think you said "no" and should have said "yes," explain why you said no and that you realize it should be yes and provide details as it relates to those questions. These things happen very often and if USCIS viewed it as a major issue, you would have received a Notice of Intent to Deny; RFE means there is a possible issue and/or misunderstanding, but they need more information/explanation.
If you are at all hesitant about your answers and the potential consequences, get an attorney's assistance (if for nothing else, peace of mind and advice you can actually rely on).
Generally speaking, you should submit a sworn statement, signed before a notary, that gives details of your military service. You may also want to include in your statement an explanation for why you answered no to questions that should have been answered yes, and that you now wish to amend those answers on the I-485.