Which of the following is a hand signal for requesting rescue boat assistance?

Waving both arms is a widely recognized hand signal used in emergency situations to attract attention and request assistance, particularly from rescue boats. This signal is effective in drawing the attention of rescuers due to its distinctiveness and visibility from a distance. It indicates an urgent need for help, which is critical when someone is in distress, especially on water where visual communication is essential.

In contrast, other choices do not convey the same urgency or clarity for requesting rescue boat assistance. Raising one arm typically serves as a general signal for attention but may not be as effective in conveying a desperate situation. Crossing arms can imply a different message, often indicating "no" or "stop," which may lead to confusion instead of signaling for help. A thumbs up is commonly understood as an indication of being okay or having no problems, which contradicts the need for rescue. Therefore, waving both arms stands out as the most appropriate and understood signal for requesting help in a rescue scenario.