Consider the following answers

Drive normally and ignore the overtaking: Not good, since the situation may be dangerous
Drive in the middle of your lane and beep or flash your full beam headlights: This is an aggressive option, which is never good in traffic. It also does not seem that the car that is overtaking is performing a dangerous manoeuvre since the car is not crossing over into your lane.
"To avoid traffic accidents, a road user shall observe the care and caution required with respect to the circumstances."
Keep to the right of your lane and be prepared to act: Good, this creates a good safety margin while preparing you for action if something should go wrong.
Reduce your speed and drive out on the hard shoulder: No need to reduce your speed only if you see that the car over taking is not able to pass you and there is an oncoming car. Only drive out onto the hardshoulder if it facilitates a safer overtaking but should not put yourself or other users in danger