Which of the following is NOT part of the mandatory vest items?

The selection of multi-tool device as the correct answer highlights that it does not fall under the category of mandatory vest items. Mandatory vest items typically consist of tools and equipment that are essential for survival and communication in emergency situations, particularly in aviation or maritime contexts.

A signal mirror, landmark compass, and MK-124 distress signal are all critical for navigation, signaling for help, and enhancing safety during unexpected situations. The signal mirror can be used to reflect sunlight to attract attention, the compass is vital for proper orientation when lost, and the MK-124 distress signal serves as a reliable means of signaling distress in water or on land.

In contrast, while a multi-tool device can be extremely useful, it is not considered essential equipment in the same way that the other items are. Thus, it does not meet the criteria for mandatory items required for safety vest protocols. The focus on life-saving and signaling capabilities helps clarify why multi-tools, although beneficial, are not designated as necessities.