The most accurate formal definition of money among the options provided is:
b. An asset that is generally accepted in exchange for goods and services and/or repayment of debt.
This definition encompasses the broader functions of money, including its roles as a medium of exchange, a unit of account, and a store of value. While the other options touch on aspects of money (such as currency and government backing), option b captures the essence of what money is meant to do in an economic context.