In which of the following cases are you required to give short horn blasts while driving a bus in the fog?

According to the regulations, in conditions of reduced visibility, such as the dense fog visible in the picture, the driver is required to exercise special caution. The regulations also impose an obligation to give brief audible signals when overtaking or passing outside a built-up area. The purpose of such a signal is to warn other road users about the manoeuvre being performed, which they may not be able to see due to the fog.