The extended metaphor in the sonnet most closely aligns with option 2: Love is only true if it is unwavering in the face of adversity, like the unmoving position of a star in the sky.
In this sonnet, Shakespeare emphasizes that true love remains constant and steadfast, regardless of external changes or challenges (represented by "tempests"). The references to an "ever-fixed mark" and the "star to every wandering bark" illustrate the idea that true love provides guidance and stability, contrasting with temporal changes and challenges (like the passing of time, symbolized by "Time's fool"). Thus, the central message is that genuine love endures and does not waver in difficult circumstances.