The correct answer is: D. full-load current is not supplied to the transformer.
During a short circuit test, a small voltage is applied to the primary winding of the transformer, causing a large current to flow through the secondary winding. This large current causes the iron core of the transformer to heat up, but because the current is not high enough to cause saturation, the iron losses are negligible.
Option A is incorrect because the current on the secondary side is actually quite high during a short circuit test.
Option B is incorrect because the voltage on the secondary side does vary during a short circuit test. The voltage is proportional to the current, so as the current increases, the voltage also increases.
Option C is incorrect because the voltage applied on the primary side is not low during a short circuit test. The voltage is actually quite high, in order to cause a large current to flow through the secondary winding.