We know that,

Sum of two adjacent angles is equal to 180^o

∴∠ A + ∠ B = 180^o

According to the condition given in the question, we have

∠ A = x° then ∠ B = 2/3 x°

∴ x° + 2x/3 ° = 180°

5x/3 ° = 180°

⇒ x =

⇒ x = 540°/5

⇒ x = 108^o

∴∠ A = 108^o and,

∠ B = 2/3 × 108°

∠ B = 2× 36° = 72°

Thus, the smallest angle = ∠ B = 72^o

Hence, option (C) is correct