What is the order of operations?

A. A set of properties that can be used to change the order in which operations are performed within expressions.
B. A set of guidelines that describe the best order for terms to be written within equations so that the equation is easy to read and understand.
C. The group of mathematicians that hold meetings to discuss the order in which expressions should be evaluated.
D. A set of rules that define the order in which the different operations should be performed so that expressions can be evaluated consistently.