The answer is a. Nitrogen
An independent variable is a variable which change is not affected by that of another variable. By process of elimination you can take out B and C. An independent variable can also change so that takes out A. That leaves D.
Newton’s first law of motion? May be wrong
Must have more protons. Protons are positively charged, so if the atom (or in this case ion) has a net positive charge, it must have more protons (+ charge) than electrons (- charge).