Which best describes a opportunity cost

Opportunity cost: The loss of a potential gain from other alternatives when ones alternate is chosen!  

Opportunity cost is an economic term used to describe a benefit, profit, or value of something that must be given up to adquire or achieve something else. This type of costs are not written down or recorded in the account book, but they are important in the process of taking a decision involving the resulting profit or loss. It is based on the idea that every resource can be used for lutliple things.

We use this concept every day without noticing, for example: You have some savings and you want to buy a new cell-phone or a new bike. The cost of buying one of them implies not buying the other one. You need to compare them and see which one gives you less losses when discarding it.

what is the tax base for the sales tax?

it should be (usually ) 6%