What auxiliary device is used in a field protective mask for Marines who wear glasses?

The correct choice, optical inserts, refers to specialized devices that enable Marines who wear corrective lenses to effectively use their field protective masks. Optical inserts are designed to fit into the mask's eyepiece, providing the necessary vision correction while ensuring a proper seal and maintaining the mask's protective features against environmental hazards. This is crucial, as a proper seal is vital for the effectiveness of the mask in preventing the inhalation of harmful substances.

In contrast, other options such as vision enhancing goggles and adjustable frames do not provide the necessary corrective vision features that optical inserts do. Protective lenses may offer additional eye protection but do not address the specific needs of Marines who require vision correction in their masks. Therefore, optical inserts are essential for balancing both vision correction and protection in a combat environment.