The path of a magnetic flux in a transformer should have

high resistance
high reluctance
low resistance
low reluctance

The correct answer is D. low reluctance.

Reluctance is the opposition to the flow of magnetic flux. The lower the reluctance, the easier it is for magnetic flux to flow. In a transformer, the path of magnetic flux should have low reluctance so that the magnetic flux can flow easily and efficiently.

High resistance would cause the magnetic flux to flow more slowly and with more difficulty. This would reduce the efficiency of the transformer.

Low resistance would allow the magnetic flux to flow more easily and quickly. This would increase the efficiency of the transformer.