A block attached to a spring, pulled by a constant horizontal force...

A block attached to a spring, pulled by a constant horizontal force, is kept on a smooth surface as shown in figure. Initially, the spring is in the natural length state. Then the maximum positive work that the applied force \( F \) can do is (given that string does not break)
(1) \( \frac{F^{2}}{k} \)
(2) \( \frac{2 F^{2}}{k} \)
(3) \( \infty \)
(4) \( \frac{F^{2}}{2 k} \)