I think 3 and 4 are reasonable-ish options.
But I do think this is a very difficult one. We are a carless family and although we are happy to walk a couple of miles, when the DCs were buggy age we did sometimes use buses for longer journeys. Sometimes if a DC was asleep and/or the buggy was laden down with shopping and extraneous bumpf it would have been very, very difficult for myself or DW (alone) to collapse the buggy, keep hold of the baby and attend to additional bags too. It relies on other passengers being helpful which is not always the case...
Recently the DCs and I were on a small bus on an hour long journey (certainly not walkable by anyone's standards). No problem for us as DCs are now well beyond buggy age. But it was a grotty, rainy October day and we went through a town centre mid Saturday pm. At three successive bus-stops three Mums with buggies got on (sensibly beating the crowds at the main town-centre bus station). I was astonished that the bus-driver allowed more than one on TBQH. It was already a full bus. I joked to the children "all we need now is for someone in a wheelchair to get on" and lo and behold at the next stop a disabled person and companion did get on. The Mums with buggies manoeuvred their buggies out of the allocated space quite civilly but in doing so did effectively blocked access/exits for a lot of the passengers (which I'm sure was a health and safety breach). I could not believe that the driver just got on with the journey and didn't demand in a jobsworth manner that the Mums get off. All were happy although I'm not sure what would have happened if there'd been an accident or some bus inspector had got on...
But I would say it's equally discriminatory to turf parents off buses if they can't fold their buggies etc.... I know it's not the same as being disabled but I'm sure most Mumsnetters can remember how shattered you can be going out with babies/toddlers (and it's not as if shopping trips into town centres/hospitals etc...aren't necessary). Can you imagine having to wait an extra twenty minutes or half an hour (as with the bus we were on) with a fretful, tired, hungry baby/toddler and laden down with shopping.
If you are generally a car-free family you might have a rather robust buggy to allow you to walk a lot with the baby. You are not necessarily going to have a fully-foldable stroller too....
Whilst most able-bodied parents would probably not mind being turfed off (or not allowed on) a bus if a disabled person needed the space, it might not be as straight-forward a scenario as that. What happens if it's the last bus on a route, it's dark and you are waiting for the bus (or are turfed off it) in a not very nice area/in grotty weather etc....
Maybe there should be some system whereby disabled passengers at bus-stops can give advance warning that they intend to get on a bus so that parents with buggies are given notice...
I am wondering what used to happen before the disabled spaces were incorporated into buses? I certainly recall that the old Routemasters had no space for wheelchairs or occupied buggies...
Have said all this, I only recall a couple of occasions when we have travelled on a bus and this has ever been an issue for us. If some bus-routes have a lot of wheelchair and buggy users maybe there needs to be a revision of the type of bus provision offered in those areas? Surely it is equally wrong to condemn either group of people (particularly parents who are isolated and car-less) to restricted lives to the advantage of the other group?
And by the way why should parents use their child benefit to get a taxi? I'm pretty sure that George Osborne would take it away from all parents if it was being used on 'luxuries' such as taxis. Haven't used a taxi anywhere since we've had children car or no!