Final answer:
The correct hand signal for stopping or slowing down is the left arm extended out the window with forearm and hand pointed down.
Step-by-step explanation:
The hand signal for stopping or slowing down in traffic is d. Left arm extended out the window with forearm and hand pointed down. This is a universally recognized signal that drivers and cyclists can use to indicate they are reducing their speed or coming to a stop when the vehicle's brake lights are not visible or in situations such as cycling where there may not be brake lights. It is important for safety on the road to understand and correctly use these signals to communicate with other road users.