State functions are properties that depend only on the current state of the system and not on how the system reached that state. They are independent of the path taken to reach the state and only depend on the initial and final states.

In this case, the question asks which of the following is NOT a state function. Let's analyze each option to determine if it is a state function or not:

a) q (heat transfer): Heat transfer is a state function because it only depends on the initial and final states of the system. The amount of heat transferred does not depend on the path taken to transfer it.

b) w (work done): Work done is NOT a state function because it depends on the path taken by the system to reach a particular state. The work done can vary depending on the process used to reach the state.

c) q_rev/T (heat transfer at constant temperature): This is a state function because it depends only on the initial and final states of the system. The amount of heat transferred at constant temperature does not depend on the path taken.

d) PV (product of pressure and volume): This is a state function because it depends only on the current state of the system, which is characterized by its pressure and volume. The product of pressure and volume does not depend on how the system reached that state.

Therefore, the correct answer is option b) q (heat transfer). Heat transfer is a state function, and the question asks for the option that is NOT a state function.