The correct answer is C) Holding the GMDSS Radio Maintainer license is sufficient. To become a GMDSS Radio Maintainer, the only requirement is to hold the GMDSS Radio Maintainer license itself. There are no pre-requisites such as holding a Marine Radio Operator Permit or a GMDSS Radio Operator license for a specific duration. The GMDSS Radio Maintainer license is a standalone qualification that certifies the holder has the necessary skills and knowledge to maintain and repair GMDSS equipment. The other options are incorrect as they impose additional requirements that are not necessary for obtaining the GMDSS Radio Maintainer license.