X decays into Y. Therefore, as time progresses, the mass of X will decrease and mass of Y will increase.
Half-life of X is 20s. After every 20s, mass of X will halve whilst mass of Y will double.
The question is asking us to identify how the mass of Y varies with time.
Graphs A, C and E show that as time progresses, mass decreases. Hence these are wrong.
Since all graphs show that there are 100g of X; after 20s, there should be 50g (half of 100g).
Mass at 20s = 50g (Correct)
Mass at 20s = 75g (wrong)
Mass at 20s = 100g (wrong)