The correct answer is A) fastened to some substantial part of the vessel. When launching an inflatable life raft, the operating cord should be fastened to a substantial part of the vessel to ensure the raft is deployed properly and remains attached to the vessel. This prevents the raft from drifting away from the vessel in an emergency situation. The other options are incorrect because: B) securing the cord to the hydrostatic release is not recommended, as the release mechanism should remain unobstructed; C) fastening the cord to the raft container could interfere with the raft's deployment; and D) leaving the cord unfastened could result in the raft drifting away from the vessel.