According to the definition of Kelvin scale,

From the above equation, it is clear that the heat transferred isothermally between two adiabatic bodies’ decreases if temperature decreases. Here  is the thermometric property.

Therefore, from above equation if  is having smaller value then T also lowers. The smallest possible value of Q is zero and corresponding T is zero.

Thus, the temperature at which a system undergoes a reversible isothermal process without transfer of heat is called as absolute zero.

Hence, the correct option is (A).