Which is the best way to solve the equation x^2 = m?

The answer would be D.

The reason so can be determined by taking a look at each option.

Option A would get us $x^4 = m^2$, and wouldn't help us in anyway.

Option B would get us ${x^2\over m}=1$, and would only get us what the value of $x^2\over m$, but that isn't what we are looking for.

Option C would get us ${x^2\over2}={m\over2}$, and would get us nowhere.

Option D would get us $x = \sqrt{m}$, 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 $x^2 = m$.