Which of the following items is found in the convenience kit pertinent to aircraft safety?

Flex cuffs are indeed an item relevant to aircraft safety, particularly in managing situations involving unruly passengers or in cases where restraint may be necessary for the safety of the crew and aircraft. They enable flight attendants to react effectively in emergency scenarios that may involve a need to secure an individual without causing harm.

The other items, although they have their places in safety protocols, do not align directly with what is commonly found in a convenience kit. Life vests are standard safety equipment but are typically stored in specific locations onboard for easy access in case of emergencies, rather than being categorized under convenience items. Similarly, while water bottles and emergency blankets are important for passenger comfort and emergency preparedness, they are generally not considered part of a convenience kit focused specifically on safety measures.