As I have mentioned in my previous thread, if the Officer-In-Charge is not satisfied with your reasons, he may ask for surety. The same (surety bond) you can get from Notary Public. You need to clarify your doubts from the Officer-In-Charge first what exactly he wants like -"should have a basic salary more than 10% of the person whose NSC is lost". For this what documents he want. So check with the post Office - Officer-In-Charge and get the job done. I am from Delhi, so no idea about Mumbai.