Q.D. A structure containing a C=O group and a structure containing a C-OH with a C=C, printed in the source only as the fragments 'O' and 'OH' respectively.

Note on the source text. Only the bare fragments 'O' and 'OH' survive extraction for this pair, so the exact printed structures are read with some inference; the answer given follows the chapter's own repeated generic keto/enol diagram (section 14.5.1(e): $-CH(-)-C(=O)- \leftrightarrow -C(=)-C(-OH)-$), which is the most standard, well-established pairing this kind of minimal 'O' vs 'OH' cue would represent in this exact context (immediately alongside parts A-C, all of which test the five named types of structural isomerism from section 14.5.1). …

Matching the minimal available cue against the chapter's own explicitly-stated generic keto/enol tautomer pair (section 14.5.1e), while disclosing the read …

• Presenting this answer with unwarranted certainty when the source fragments are genuinely too sparse to fully confirm the exact structures -- the reasoning here rests on pattern-matching to t …