Erutepa is correct in that this isn’t relevant to VCE Biology. I know this stuff used to be more relevant to VCE (action potentials used to be a part of the study design!) but they were removed before my time.
I believe the answer would be
Neuron A has more K+ channels open than Neuron B. Your line of thinking was close, but we have to take into account that the permeability of K+ is roughly 20 times more than the permeability of Na, so it’s a much stronger determinant when calculating RMP.
We can actually see how the permeability of K+ affects the resting membrane potential by calculating the resting membrane potential ourselves. If you’re curious, you can see it in
this video by watching from the start up to 6:45, which goes through something called the Goldman-Hodgkin-Katz Equation — it'll do a much better job than what I could. Naturally if it’s a bit confusing, that’s okay, because like Erutepa said this is not relevant to the study design.