Q.The reaction of ethylene glycol with $PI_3$ gives :

Periodic acid cleaves the C–C bond of the vicinal diol $HO-CH_2-CH_2-OH$, converting each $-CH_2OH$ carbon into $-CHO$, so the product is methanal (HCHO); $HIO_4$ itself is reduced to $HIO_3$.

The general reaction of a 1,2-diol with periodic acid is $R_2C(OH)-C(OH)R_2' + HIO_4 \rightarrow R_2C=O + R_2'C=O + HIO_3 + H_2O$, cleaving the bond between the two hydroxyl-bearing carbons. For ethylene glycol, $HO-CH_2-CH_2-OH$, both carbons are $-CH_2OH$ (each bearing 2 H, 1 OH, and the C–C bond to be cleaved), so cleavage converts each into $H_2C=O$, i.e. methanal. No further oxidation to methanoic acid or $CO_2$ occurs under these specific cleavage conditions, and glyoxal (OHC–CHO) would only …

Ethylene glycol + $PI_3$ forms the vicinal diiodide as an unstable intermediate, which immediately eliminates $I_2$ to give ethylene gas.

$PI_3$ replaces $-OH$ by $-I$, so the initial product of $HOCH_2CH_2OH + PI_3$ is 1,2-diiodoethane, $ICH_2-CH_2I$. However, vicinal (1,2-) diiodides are thermally and chemically unstable at ordinary temperature because the weak C–I bonds allow easy loss of iodine; the diiodide spontaneously decomposes, releasing $I_2$ and regenerating the alkene, $CH_2=CH_2$. So while $ICH_2-CH_2I$ (option a) is the fleeting intermediate, it is not the isolable product — …