Which one-line statement best describes the Square Root Property?

Whenever you have a quadratic equation you must factor it first, then solve for each factor that is set equal to zero because factoring is the only reliable method for all quadratics.

If $(\text{expression}{)}^2$ equals a constant, then taking both the positive and negative square roots of that constant will give solutions that, when substituted back, are guaranteed to satisfy the original equation because squaring either sign yields the same positive result.

When a squared expression equals zero you write the single solution as a positive number only because negative roots are never allowed for squared equations.

If $a^2=k$ then $a=\pm \sqrt{k}$.