What signal must you give a policeman when you are about to go straight ahead?

The correct signal to give a policeman when you are about to go straight ahead is to raise your hand towards your shoulder and then move your forearm well forward and back in a vertical plane. This gesture is a universally recognized form of signaling, providing clear visual communication that you intend to proceed in a forward direction.

This method is particularly important as it is highly visible and can be easily understood by the policeman directing traffic. Proper signaling is critical for maintaining safety on the road, ensuring that all vehicles and pedestrians are aware of the driver's intentions.

In contrast, the other options do not effectively convey the intended message. Extending both arms sideways is typically used for signaling a turn, while pointing straight ahead with a finger lacks clarity and may be easily overlooked in traffic. Flashing your headlights might communicate a different message, often associated with alerting other drivers rather than indicating a specific direction of movement in relation to traffic control by an officer.