The transformers use alternating current, so there will be the necessary
Answer 1
modification of the magnetic field for operation
answer 2
A little vague question, but I think you are asking why AC power is needed for a transformer. the basic answer is that it provides a rapidly changing input current, resulting in a changing magnetic field (flux). This makes the 3rd and 4th answers the correct ones. All the other answers are related to the transformer, but not specifically to the AC requirement.