=== Model: grok-4.6-direct | Task: bench:gpqa_diamond | Rep: 1 ===
=== Score: 5/5 | Wall: 414.904s | Cost: $0.151740 ===
=== Pass: True | Finish: stop | In-reasoning-only: False ===

--- JUDGE ---
gpqa_diamond

--- RESPONSE ---
The compounds must be evaluated individually for chirality (the presence of nonsuperimposable mirror images, with no plane or center of symmetry). Specified stereoisomers that are chiral will be optically active; meso forms and achiral molecules will not.

- (Z)-1-Chloro-2-methylbut-1-ene has a geometric double bond but no stereogenic center and possesses a molecular plane of symmetry. It is achiral.
- (3aR,7aS,E)-8-(Chloromethylene)hexahydro-4,7-methanoisobenzofuran-1,3-dione is a norbornane-fused anhydride bearing an exocyclic =CHCl unit. The two ethano bridges are constitutionally distinct, the given absolute configuration and double-bond geometry remove all symmetry elements, and the molecule is therefore a single enantiomer (chiral).
- (2R,3S)-2,3-Dimethylsuccinic acid possesses two stereocenters of opposite configuration and an internal mirror plane; it is the meso diastereomer and achiral.
- (2R,3R)-2,3-Dimethylsuccinic acid is one enantiomer of a classic resolvable pair and is chiral.
- (R)-Cyclohex-3-en-1-ol contains a single stereogenic center (C1) whose two ring paths are constitutionally different; it is chiral.
- (1s,3s,5s)-Cyclohexane-1,3,5-triol (whether the all-cis or the 2:1 cis/trans isomer) always retains a mirror plane and is achiral.
- 1-Cyclopentyl-3-methylbutan-1-one has the constitution \(\mathrm{(CH_3)_2CHCH_2C(=O)C_5H_9}\); C3 bears two identical methyl groups and the molecule contains no stereogenic element.

Exactly three of the structures are chiral and therefore optically active.

The answer is (D)