I agree with MisterNeo, because you wanna make sure you have no regrets. You mind find you suck at one subject, or if you're good at them all, that you really despise one subject (I dropped the subject I was coming 1st in the middle of June. Yes I was acing it but I absolutley hated it and I'm so glad it's gone)
As for which ones "maximise your ATAR" it comes down to how well you do in them. I personally don't do any extension subjects but I do study bio and I think it's fabulous! Sure, bio has a lot of content at first glance, but the syllabus does break it all down quite nicely so remembering it is rather doable. Also, extension history has a major work, as you are probably already aware, so you need to make sure you are committed to it. If your committed, start early and love what you're doing and put in the hard work, it will pay off And you'll do well. If you're not as passionate it might end up being a stressful struggle, particularly near submission.
Also if you do take the advice to give the descision time, don't be scared to drop a major work subject, just because you started. The one I dropped involved a major work and the fact I don't have to finish is a weight off my shoulders in its self, but before the descision I was initially scared of sending all that work down the drain.
There's also the option of maybe doing 11 units, a nice middle that isn't as intense as 12 units and still gives you a slight backup unlike 10. So maybe you could do option a but with one of the extensions too.
I think I waffled on a bit too much, sorry, but hope this helps the decision in some way.