**Explanation:**

The efficiency of a transformer under load conditions can be determined by conducting a back-to-back test. Let's understand why this test is used to determine the efficiency of transformers.

**Back-to-Back Test:**
In a back-to-back test, two identical transformers are connected back-to-back with their primary and secondary windings in parallel. The primary of one transformer is connected to the supply, and the secondary of the other transformer is connected to a load. This configuration allows the two transformers to share the load, and the test is conducted under normal operating conditions.

**Reasons for Choosing Back-to-Back Test:**

There are several reasons why the back-to-back test is chosen to determine the efficiency of transformers under load conditions.

1. **Accuracy:** The back-to-back test provides accurate results because it is conducted under actual operating conditions, simulating the real-world scenario.

2. **Real Load Conditions:** The test allows the transformers to operate under load conditions, where the primary transformer supplies power to the load and the secondary transformer receives power from the load.

3. **Identical Transformers:** The two transformers used in the test are identical, which ensures uniformity and eliminates any variations in the results due to differences in transformer characteristics.

4. **Efficiency Calculation:** By measuring the input and output power of each transformer, the efficiency can be calculated using the formula:

Efficiency = (Output Power / Input Power) x 100%

The output power is determined by measuring the power received by the load, and the input power is determined by measuring the power supplied by the primary transformer.

5. **Load Sharing:** In a back-to-back test, both transformers share the load, which helps in distributing the power evenly and preventing overloading of a single transformer.

6. **No Additional Equipment:** Unlike the short-circuit test or open circuit test, the back-to-back test does not require any additional equipment or special arrangements. It can be easily conducted with two identical transformers and a load.

Therefore, the back-to-back test is considered the most suitable method for determining the efficiency of transformers under load conditions, as it provides accurate results and simulates real-world operating conditions.