(c) 20.8 cm
Let the edge of the single cube be ‘a’ cm.
Then, total volume melted = Volume of cube formed
⇒ (6)3 + (8)3 + (10)3 = a3
⇒ a3 = 216 + 512 + 1000 = 1728 ⇒ a = 12 cm.
∴ Diagonal of the new cube = \(\sqrt3\,a=(\sqrt3\times12)\) cm = 20.8 cm (approx.)