What is the main function of a power transformer?

A power transformer is responsible for transporting power from one part of the system to another.
A power transformer is responsible for transforming the voltage of the system from one nominal level to another.
A power transformer is responsible for protecting the system from overloads and short circuits.
a) Only statement I is correct.
b) Only statement II is correct.
c) Statement II and III are correct.
d) Statement I and II are correct.