All other factors being equal, the diffusion rate of nickel is the highest in copper with ASTM grain size number of:
Honestly, the question does not appear to be very intuitive - I've searched through my notes and cannot seem to find any relatable formulae. I understand that the number of grains per square inch at 100X magnification is equatable to 2^(n-1), where n is the ASTM grain size number of the specimen.
However, given the multiple-choice nature of this question and the apparent lack of information (I suppose the copper and nickel are pure, at room temperature, etc.), I must be overlooking something simple.
Would anyone be willing to advise?