- (a) "the temperature of the gas remain constant" - This is the definition of an isothermal process. Correct. - (b) "the gas does not take any heat from the surroundings" - This is incorrect. For the temperature to remain constant during expansion, the gas must absorb heat to do work (ΔU = Q - W; if ΔU=0, Q=W). During compression, it must release heat. Correct. - (c) "the internal energy of the gas remains constant" - For an ideal gas, internal energy is solely a function of temperature (U = U(T)). Since the temperature remains constant in an isothermal process (T=constant), the internal energy of an ideal gas also remains constant (ΔU = 0). This is a fundamental property of isothermal processes for ideal gases. Correct. - (d) "the pressure and volume of the gas remain constant" - This is incorrect. In an isothermal process involving a gas, pressure and volume typically change while maintaining a constant product (PV = constant for an ideal gas, following Boyle's Law). Only if there is no work done (W=0) and no heat exchange (Q=0) would P and V potentially remain constant, which would be an isochoric or isobaric process under specific conditions, not a general isothermal one. Correct.