The answer would be D.
The reason so can be determined by taking a look at each option.
Option A would get us x4=m2, and wouldn't help us in anyway.
Option B would get us x2m=1, and would only get us what the value of x2m, but that isn't what we are looking for.
Option C would get us x22=m2, and would get us nowhere.
Option D would get us x=√m, and get us the value of x in terms of m, which is more helpful, and since x is normally the unknown or what we are trying to solve,
Option D is the best way to solve the equation x2=m.
