Final answer:
In a steam engine, thermal energy is converted to mechanical energy. This is the core function of a steam engine, where the heat produced from burning fuel creates steam that drives mechanical work.
Step-by-step explanation:
The action and reaction in a working steam engine is best described by the transfer of energy from one form to another. Specifically, in the context of a steam engine, the correct answer is B) Convert thermal energy to mechanical energy. A steam engine operates by burning a fuel (often coal) to create thermal energy that heats water, producing steam. This steam expands and is used to do mechanical work, such as turning the wheels of a train or spinning a turbine. If the turbine is connected to a generator, it can produce electrical energy, but the primary action of a steam engine itself is converting thermal energy into mechanical energy.
Overall, the transformation of energy is a common theme in many systems. For instance, in nuclear power plants, the nuclear energy is transformed to thermal energy, which then may be converted into electrical energy. These transformations demonstrate the fundamental principle that energy can be transferred and transformed but cannot be created or destroyed.