How can crew members extend their GT-II currency?

The correct option indicates that crew members can extend their GT-II currency by qualifying for GT-V or GT-IX. This is based on the training and qualification progression within the gunnery training program. Advancing through the gunnery tiers signifies that the crew has demonstrated increased proficiency and capability, reinforcing their skills and knowledge.

Qualifying for higher tiers, such as GT-V or GT-IX, is often designed to ensure that crew members are not only maintaining but also enhancing their operational readiness and tactical effectiveness. This means that their experiences and competence gained during higher-level qualifications contribute to their overall readiness, thereby allowing them to extend their GT-II currency.

Other choices might suggest different methods to maintain currency but do not directly align with the prescribed gunnery qualification paths that are in place to ensure consistent skill enhancement and readiness. Thus, progressing to a higher qualification tier is the most effective way to sustain and extend GT-II currency.