A biphenyl is optically active when it exhibits atropisomerism. This occurs when bulky substituent groups are present at the ortho positions of both phenyl rings, which severely restricts the free rotation around the single bond connecting the two rings due to steric hindrance. As a result, the two rings become non-coplanar (often perpendicular). If each ring is unsymmetrically substituted, the molecule lacks a plane or center of symmetry, making it chiral and optically active. Since the structural options are missing from the provided text, the exact correct option cannot be visually evaluated.