Since we are missing the diagram to work with, we will answer this assuming that the test tube was placed inside a beaker of stearic acid, in which case the transfer of thermal energy is by way of radiation.
Thermal radiation is the most common method of transferring thermal energy. We see this every day, for example when a cup of coffee becomes colder over time, this is due to the coffee radiating its heat into the surrounding air. The heat will travel in waves through air or water, which is what happens to the test tube and the beaker.
Therefore we can confirm that in the case described and with the assumption made regarding the placement of the test tube, the transfer of thermal energy will be by way of radiation.
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