What item was dropped to signal the start of a race in Roman chariot racing?

In Roman chariot racing, the item used to signal the start of a race was called the mappa. This was typically a white cloth or flag that a designated official would drop at the beginning of the race, indicating to the charioteers that they could start racing. The mappa played a crucial role in the organization of the event, as the timing of its drop was essential for a fair start among the competing chariots.

The other choices do not align with the historical practices of Roman chariot racing. Parchment refers to a writing material and would not be practical for signaling a race. A token is a general term that does not specifically relate to the racing signal, and an anklet is a type of jewelry with no relevance to the initiation of a race. The mappa is specifically tied to the context of the races, making it the correct answer.