= Solution
\b[The depicted trans-fused $A$ is <chiral>; cis-fused $B$ is <achiral> as a configurational species.] The unsaturated ring makes the two sides of the fused system constitutionally different. At the trans junctions of $A$, the mirror image cannot be superposed by a proper rotation; there is no surviving mirror plane or inversion center that exchanges the two distinct rings correctly. A twofold rotational relationship is compatible with chirality and does not remove it. This is why simply importing the chirality conclusion for fully saturated trans-<decalin> would be unsafe.
For $B$, interchange of the two junction ends relates the opposite local stereochemical senses. An individual folded cis conformer may look <chiral>, but <ring inversion> interconverts its mirror-related <conformations> without changing junction <configuration>. The configurations therefore describe a meso/<achiral> species rather than a persistent enantiomeric pair. The conformational sketches below distinguish the rigid trans arrangement from the two accessible cis folds; the <alkene> ring is represented schematically as a half-chair-like unit.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2024/paper-1-decalins.png]
{title=Fused-ring conformations and the stereochemical consequences of anti bromination}
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