When electronegativity difference arises between the bonded atoms, then a molecule is polar.
When electros are shared equally between the bonded atoms or when the polar bonds in a bigger molecule cancels out each other, then a a molucule is non polar.
(a) 50% isopropanol/H2O,--- 2 (second least polar)
(b) 25% isopropanol/H2O,----- 3 (third least polar)