Correct option is B
If frequency of the transformer is decreased (assuming its applied voltage is same), magnetizing current in the primary of the transformer increases as it is inversely proportional to frequency and directly proportional to the applied voltage.
This magnetizing current is used to set up flux in core of the transformer. If magnetizing current goes beyond certain limit, the transformer core may saturate.
Therefore, it is required to reduce applied voltage along with frequency in same proportion to keep the magnetizing current same.
V/f ratio must be constant.
Here v is constant, f is decreased to half. Then Bmax increased to double, which will drive the core in to deep saturation and also Iμ is very high to create double the rated flux.
