The thermodynamic variable temperatures is defined by:
(A) Zeroth law
(B) I law
(C) II law
(D) III law

Hint: Use the Information that is obtained from all four laws of thermodynamics. Each separate law gives some specific information about heat transfer and temperature. We use a thermometer for the temperature measurement with the help of thermal equilibrium.

Complete step by step solution:
We can obtain information about heat transfer, and the changes occur in the properties of the matter due to this heat transfer from the thermodynamics studies. The properties like temperature, pressure etc. tends to change when matter interacts with heat. The changes occur because of the internal energy of the systems affected by heat transfer. If the system gains some amount of heat, then the system's internal energy increases and increases temperature and pressure. If the system releases the heat, then its internal energy decreases.

We can measure the properties of variation from the different types of devices like a barometer is used for the pressure measurement, and a thermometer is used for the temperature measurement. The thermometer working is based on thermodynamics' zeroth law because zeroth law gives information about the variable temperature.

Therefore, the thermodynamic variable temperatures are defined by zeroth law, and option (A) is correct.

Note: We get information about the internal energy of the system for the first law, and from this law, we cannot get some information about temperature variation that why option (B) is wrong. From the second law, we get information about the direction of heat transfer and the quality of heat, so that's why (B) and (C) are wrong.