Which signal must be given during the day to indicate that crew members are going between cars?

The correct answer indicates a critical safety practice in rail operations. A stop signal with an extended arm is a clear and visible way for crew members to communicate their intention to halt all train movements while they are moving between cars. This signal is essential during the day when visual communication is paramount. It ensures that anyone operating the train or working near the railcar is aware that crew members are in a hazardous position and that all movement must stop to prevent accidents.

The other signals may cause confusion or may not be universally recognized as an alert for crew members moving between cars. Therefore, using the stop signal with an extended arm provides the necessary clarity and authority to ensure safety in that situation.