JEE Main · 2021 · 26 Feb 2021 · Shift 1 · Hydrogen

Deuterium of heavy water exchanges with the labile hydrogen atoms of a substrate, and following where the label goes is a standard way of settling a mechanism, so the first statement is true.
Ordinary water is electrolysed faster than heavy water because the lighter $\mathrm{O}-\mathrm{H}$ bond breaks more readily, so prolonged electrolysis leaves a residue steadily richer in ${\mathrm{D}}_2\mathrm{O}$; exhaustive electrolysis is indeed the classical preparation of heavy water, and the second statement is true.
The heavier deuterium bridges hold ${\mathrm{D}}_2\mathrm{O}$ molecules together more firmly, so heavy water boils at $374.4$ K against $373$ K for ordinary water, and the third statement is true.
Those same stronger bridges make heavy water the more viscous liquid, so the claim that ordinary water is more viscous than heavy water is false.
The true statements are the use in exchange reactions, the preparation by exhaustive electrolysis and the higher boiling point.