If dark areas on the frame is more than 50%, I will choose under expose (a), and over expose if less than 50% (c), or (b) if 50/50 darkness and brightness. However it is very subjective. I want to get better at this so I ask 8) people may have different ideas about this though
I'd choose the lowest one I could to give me the shutter speed and/or DoF I needed, since I'll take noise over motion blur or too thin a DoF.
Note that I'm talking about the 1D X, where all of those are native ISO settings. Rather than use an expanded (H1, etc.) setting, just underexpose to meet shutter speed/aperture requirements, then push in post - that's what your camera is doing anyway.
I'd need more information as to what your talking about. Is the camera in manual? Normally, I'd use 6400 and open the lens one stop or slow the shutter one stop.
There is insufficient information to know what the situation actually is.