In absence of a death certificate Initially it has to be legally proved that A is missing. Was any missing complaint made to the Police authorities? if not there has to be a death certificate to claim his properties. Mere making a claim of the person being missing does not permit anyone to partake in his properties.
The burden of proof that a person is dead lies on the person who claims he is dead. The proof should be tangible, verifiable and conclusive. A missing person complaint filed with the police authorities and subsequent 'untraceable' report is the best acceptable proof.