Which of the following atoms would be most likely to become a cation?
a) copper
b) oxygen
c) fluorine
d) nitrogen

Copper, because it is the only metal out of all of them. Therefore copper is the only element that can loose electrons to have a positive charge, it is the most likely to become a cation.
I have not learnt this in school yet, but I researched it, and this is what all the websites say.
Hope this helps!

Rate law for this reaction israte of the reaction = k[a]²[b].second order in a  means concentration of a in power of two, and  first order in b means concentration of b in power of one.