Its not so much of an issue for mountain bikes, as we have less options when we go out and about, as bridleways are much less in number than roads, even more so, interesting ones worth going out for.
but road riding, i imagine the options to be too numberous, and the navigational choice to be a bit of a hastle?

When you go for a road ride do you:

a) Stick to a known, regularly ridden route
b) Go out with a destination in mind, and just work it out as you go
c) Make it up as you go along, go with the wind.

A mix of all three along with
4) sit down with a map and plan a route
5) do a Sportive or a road race where I don't have to worry about such things!

Bit of all 3 normally.

Make it up as I go along, eventualy you learn enough of the local roads to string together enough ridning without repeating sections or straying so far form home that a taxi ride/lift would be out of the question.

The alternative is set out with a destination and follow road signs. Done a few big rides this way, pick 2 points, make a mental note of the vilages in between and follow the road signs. Then get the train back. Feels quite good to do 100mile rides then run out of country as you've reached the sea.

Yep, as above.

As I (and I expect most people) nearly always ride from home I know where pretty much every little lane goes, so it's more or less all familiar, just depends on the order I ride them in, which is dictated by how long I want to be out for and the terrain I fancy! I've also done Crazy-Legs number 4 on occasion when doing a longer route.

I guess you only get to know the roads by following your nose though, so that comes first!

Yep accumilation of rides over the years so you get to know each and every road.

If I want to try a new area I check a map and put it down to memory or do a little card. If it all goes wrong you're on the road so I follow my nose or signs.
TBH it's something I've never actually given thought to, just happens.

Normally stick to a regular route with a few detours depending on how I feel, though did go on a bit of a random "ooh, wonder where that road goes" on Monday with a vague knowledge that I just needed to turn left at some point to pick up a known road/village again if I needed it.

Managed to discover some nice little roads before I'd realised I'd put 25 miles onto the route (must get computer back on bike…) and got a strict talking to from Mrs Breathe for being so late back.

Usually time is an issue so I do combinations of roads I know rather than exploratory wanderings, taking in new bits if I have a pretty good idea where I'm going to come out.

As others have said it's a mix of all three. Some rides I plan an exact route but thats only if I'm going to a specific destination, most rides I head out the door with a rough route in mind but no set route plan and a few rides I just go out and see where it takes me.

Pick a cafe, see who is around to meet up for the ride, see how we are all feeling on the day, then decide how best to get to the cafe.

I have a book describing local tours (it's not where I grew up so I don't know the roads that well, and I don't have a car so only really travel about on my bike) that I mark on a map which I just stick in my back pocket. I was a bit cynical at first – the book was a present – but the routes are just great and not really where I would have thought of going myself. But I also do quite a lot of a and c.

4, a, and b, b being supplemented by a knowledge of local roads i.e. "Well I go out towards suchandsuch but I'll take the back road cos it's quieter etc"

I often put routes on my Garmin Edge 305 cos then I can do them again and compare times.