A disccount is a value that needs to be subtracted by the original cost of the product. The original cost is the base that we will use and represents a total of 100% or 1. So we first need to subtract the total discount given to the product, in this case it is 20% or 0.2. We then need to calculate the value of the tax, this is based on the ammount paid for the product, in other words it must come after the discount. A tax adds a value on what we would pay otherwise, so if the base is 100% or 1, we need to add the tax which is 6.5% or 0.065. With this we will have the following expression:
The correct alternative is the third one.