Transformer A with 4% voltage regulation is better.
Justification: Voltage regulation refers to the percentage change in secondary voltage when load is changed from no-load to full-load with primary voltage kept constant. Ideally the secondary voltage should remain constant in spite of change in load. However, in practice, due to winding resistance and reactance, the secondary voltage changes with change in load. Therefore, the transformer with minimum change in secondary voltage is preferred. Since transformer A has 4% voltage regulation, which is less as compared to 6% of transformer B, it is better.