=== Model: grok-4.6-direct | Task: bench:gpqa_diamond | Rep: 1 ===
=== Score: 5/5 | Wall: 20.859s | Cost: $0.010702 ===
=== Pass: True | Finish: stop | In-reasoning-only: False ===

--- JUDGE ---
gpqa_diamond

--- RESPONSE ---
The two compounds given for tautomerism analysis are constitutional isomers of one another (both \(\mathrm{C_6H_4O_2}\)). Cyclohexane-1,3,5-trione readily undergoes keto–enol tautomerism to the aromatic triphenol phloroglucinol. In contrast, \(p\)-benzoquinone lacks abstractable \(\alpha\)-hydrogens that would permit an analogous proton shift while remaining an isomer; the related hydroquinone differs in molecular formula and is therefore not a tautomer. Hence the compound that does not exhibit tautomerism is benzoquinone.

Of the two esters given for stereochemical analysis, methyl 2-hydroxypropanoate (methyl lactate) possesses a stereogenic carbon atom bonded to four different substituents (\(\mathrm{H}\), \(\mathrm{OH}\), \(\mathrm{CH_3}\), \(\mathrm{CO_2Me}\)) and is therefore optically active. Dimethyl fumarate is a planar, centrosymmetric alkene that possesses neither a stereogenic center nor any other element of chirality. Hence the compound that exhibits optical isomerism is methyl 2-hydroxypropanoate.

These two assignments correspond to option D.

The answer is (D)