Exit
which of the following is an example of an informal economic institution?
a. tipping
b. a credit union
c. a real estate office
d. a savings and loan institution

I would say that a.tipping is the correct answer

on the one hand, all the other options are formal and require a formally written agreement between all sides, and on the other hand tipping also proceeds very informally, for example by just leaving money in a jar or on the table.