Based on the information provided, it appears that the gas undergoes an isothermal change during the time in question. An isothermal change is a type of thermodynamic process in which a system undergoes a change in temperature, but the temperature remains constant. In this case, the gas is supplied with 3500 J of thermal energy, which means that it gains heat. At the same time, the gas does 3500 J of work on its surroundings, which means that it expends energy. Since the net change in the gas’s energy is zero (3500 J gained - 3500 J expended = 0), the temperature of the gas must remain constant throughout the process. Therefore, the correct answer to the question is option B, isothermal.