In an isothermal change, a thermodynamic process occurs at a constant temperature. This means that the temperature of the system remains unchanged throughout the process. For this to happen, heat must be exchanged with the surroundings to compensate for any work done by or on the system. For example, in an isothermal expansion, the system absorbs heat from the surroundings to maintain its temperature while doing work on the surroundings. Conversely, in an isothermal compression, the system releases heat to the surroundings as work is done on it. This type of process is often idealized in the study of gases, particularly in the context of the ideal gas law, where the product of pressure and volume remains constant if temperature is constant. Thus, in an isothermal change, the defining characteristic is that the temperature remains unchanged, making the correct answer [B] Temperature remains unchanged.