It is given in the question that,

In , ∠A = 40^o

∠B = 60^o

We know that, sum of all angles of a triangle is equal to 180⁰

∴ ∠A + ∠B + ∠C = 180^o

60^o + 40^o + ∠C = 180^o

∠C = 180^o – 100^o

∠C = 80^o

Hence, the side which is opposite to ∠C is the longest side of the triangle

∴ Option (C) is correct