Explanation:

Let say average wt of 9 Mangoes = x gm

weight of 9 Mangoes = 9 x  gm

Let say weight of mango which was replaced by 120gm mango = M gm

New weight of 9 Mangoes = 9x + M - 120 gm

New Average = x + 20 gm

9 (x + 20) = 9x + M - 120

= 9x + 180 = 9x + M - 120

= M = 300 gm

Hence, option 4 is the correct answer.