Final answer:
The correct answer is that the lead rider is signaling to slow down or prepare to stop. Hand signals are a vital part of motorcycle group riding safety.
Step-by-step explanation:
When a lead rider extends their left arm straight out with their palm facing up before swinging their arm upward, it signals that the rider intends to slow down or is preparing to stop. This is a common hand signal used by motorcyclists to communicate with each other when riding in groups. It is important for all riders within the group to understand and use these hand signals to ensure the safety of everyone on the road.