Problem: Determine the correct category for the following items a to d and explain your rationale (Unrestricted, Temporarily Restricted, and Permanently Restricted Net Assets):
a) Revenue from gift shop sales
b) Donor gives cash to the organization, specifying it cannot be used until the following year
c) Donor makes a pledge for current year operations but it is not yet paid
d) Donor gives organization a statue, but makes it promise to only display the statue and not sell it.