How do you respond when another driver tailgates you? (C’mon, be honest)?

You brake check them.  See how fast they back off.

This seldom happens because when I do drive it's a 4x4 very obviously used for trade so people must expect that the driver is big and aggressive.  They're wrong on the first point and mostly wrong on the second.  The last time I was driving a small car and an idiot tried to play that game I slowed down so much that he had to overtake if he wanted to prevent an accident and then I set about trying to run him off the road.  I'm not as ashamed of that as I should be because I don't think that idiot will have tried it again but I did realise that I was so angry that my hazard perception had completely evaporated and I would've happy for him to go up in a fiery ball and me too, so I went and got some anger management therapy.  I must stress that my reaction was completely out of character which is what rattled me.  

I usually slow to a speed where I can be sure they can stop before hitting me, if I have to make a sudden brake application.

I was a school bus driver. We were taught to pull over and let them pass. But as a private driver, I have done all three of your choices. 

I used to be a 3 but as I've aged I've turned into the 1.   Maybe someday I'll get to the 2, which is where I wish I was. 

There is no way I can say that only one applies!

Often I just ignore them and when a passing zone comes up, I will let them pass and do my best to make sure they can do so safely (slow down and some times even brake so they can get over).

It isn't till they refuse to pass that it starts to bother me. If it goes on long enough often I see just how fast they are willing to go to stay on my back bumper. Provided I know the roads and the situation is such that I feel it is safe to do so.

If it isn't, I find a place to get off the road and take a break.