To warn other drivers, the signal is given in advance, before the maneuver begins. A bus moving along its established route is no exception.
Under clause 36, before starting any maneuver (for example, changing lanes), the driver must signal in advance with the turn signal lights of the relevant direction. This warns other road users in time, so that they can react safely.
Option 1. Signaling immediately before the maneuver does not give other drivers enough time to react.
Option 2. Signaling after the maneuver has started defeats the requirement to warn other drivers in advance.
Option 4. Public transport vehicles are never exempt from the mandatory rule to signal before changing lanes.