Answer: B. the isovolumetric process
Explanation:
In the graph given, the volume is constant throughout. It represents a constant volume process. Such processes are called the isovolumetric process or isochoric process.
Hence, option B is the correct answer.
Option A is incorrect because in an isobaric process, the pressure is constant.
Option C is incorrect because in an isothermal process, the temperature is constant.
Option D is incorrect because in an adiabatic process there is no heat transfer.