"There ain't no such thing as a free lunch" expresses the idea that even if something seems like it is free, there is always a cost, no matter how indirect or hidden.
The correct answer is: "everything has a cost, if not a explicit one at least a opportunity cost".
The opportunity cost has to be considered when making a decision. It is defined as the value of the best alternative rejected when choosing a certain option. This concept shows that every good/service, even when it is consumed at a zero price, entails a cost. For example, there is a free concert in the street, but the time spent on its consumption is a resource that could be dedicated to an alternative activity that has a value and such value is the opportunity cost.
