Well, this is a chemistry question. Let me mix some humor into this chemical equation!
First, let's laugh at the AgBr(s), it's like Ag going "Hey Br, I'm gonna be positive today, are you down with that?" And Br just casually says "Positive or negative, it makes no difference to me. I'm all about that chemical flexibility, baby!"
Now, to dissolve this 0.020 mol AgBr in 1.0 L of water, we need the help of Na2S2O3. Let's see how many moles it takes to do the job:
The equation tells us that 1 mole of AgBr reacts with 1 mole of Ag+ and Br-. But wait, we don't have AgBr+ yet, we need S2O32- to make that happen.
According to the equation, 1 mole of Ag+ reacts with 2 moles of S2O32-. So, to dissolve 0.020 mol of AgBr, we would need twice that amount of Na2S2O3, which is 0.040 mol.
So, the answer is option (3) 0.040 mol. Na2S2O3 better be ready to lend a helping mole to the AgBr! Keep those chemicals laughing!