The correct answer is: B. back-to-back test
A back-to-back test is a method of testing the efficiency of two identical transformers under load conditions. The two transformers are connected in parallel, with their primary windings connected to the same source voltage and their secondary windings connected to the same load. The current and voltage in each transformer are measured, and the efficiency is calculated as the ratio of the output power to the input power.
A short-circuit test is a method of testing the impedance of a transformer. The transformer is connected to a short circuit, and the current and voltage are measured. The impedance is calculated as the ratio of the voltage to the current.
An open circuit test is a method of testing the open circuit voltage of a transformer. The transformer is connected to an open circuit, and the voltage is measured. The open circuit voltage is the maximum voltage that the transformer can produce.
Neither a short-circuit test nor an open circuit test can be used to determine the efficiency of a transformer under load conditions.