This question would be easier to answer if you attached a picture of the graph the question is referring to.
There's two sig figs: the 1 and the zero. The 10^-2 contributes no sig figs.
Potassium is more reactive because it is in group one
To balance it, it would be N2 + 3H2 ------> 2NH3.
for c) it would be 2N2 + 6H2 -------> 4NH3