Oxidative phase of glycolysis starts with dehydrogenation of:

Explanation

In the oxidative phase, each molecule of glyceraldehyde 3-phosphate (produced earlier in the preparatory phase) undergoes dehydrogenation. This involves the removal of a hydrogen atom from glyceraldehyde 3-phosphate, which is transferred to a molecule of NAD+ (nicotinamide adenine dinucleotide) to form NADH. The resulting molecule is 1,3-bisphosphoglycerate.