Which of the following cannot be explained by the open chain structure of glucose?

Glucose exists in two forms in nature; $α$ and $β$.

The correct answer is Option (4) → Glucose exists in two forms in nature; $α$ and $β$.

1. Glucose reacts with hydroxylamine →  can be explained by open-chain aldehyde group (forms oxime).
2. Glucose undergoes oxidation with nitric acid →  can be explained by aldehyde and –CH₂OH groups in open chain form (forms gluconic and glucaric acids).
3. Glucose reacts with acetic anhydride to form pentaacetyl glucose →  all –OH groups in open-chain form can be acetylated.
4. Glucose exists in two forms in nature; α and β →  cannot be explained by open-chain structure, because α and β forms are cyclic (pyranose) forms resulting from ring closure (hemiacetal formation).