The best equation would be A.
with A you get to calculate what is the value of $.
8 + $ = 10 (10 is the total value of your receipt and 8 is the price of pizza)
$ = 10 - 8 (we transfer the price of pizza on the other side to the total amount of your receipt and calculate the remaining amount)
$ = 2
We see that the remaining amount is 2$, which means the salad cost s2$ :)