A triangle is not said to be a right-angled triangle if its sides measure:

20 cm, 21 cm, 28 cm

All are multiples of common triplets except option 3:

i)  1.5, 2, 2.5 = 0.5 × (3, 4, 5)

ii)  18, 24, 30 = 2 × (9, 12, 15)

iii)  20, 21, 28 = (20, 21, 29)

iv)  2.5, 6, 6.5 = 0.5 × (5, 12, 13)

Hence, option 3, is not sides of right-angled triangle.