Geo-Scientist Prelims 2022 Chemistry Q118: The intermediate which is less likely to be formed in the mechanism…

The intermediate which is less likely to be formed in the mechanism of the following reaction is

1. (a) Secondary cation at C3 (CH₂OH retained)
2. (b) Tertiary cation at C2 (CH₂OH retained)
3. (c) Protonated aldehyde (cation on the CH–OH carbon, C1)
4. (d) Primary cation at C1 (H₂C⁺) with the C3–OH retained