Two metal strips that constitute a thermostat must necessarily in their (A) Mass (B) Length (C) Resistivity (D) Coefficient of liner expansion

Let's first look at the role that each of the properties plays in the functioning of a thermostat: (A) Mass - This refers to the amount of matter in the metal strips. It does not have a direct role in the functioning of a thermostat. (B) Length - The initial length of each strip may vary but it is not a necessary condition for the thermostat to work. (C) Resistivity - This refers to the electrical resistance of the metal strips. Although a thermostat can use changes in resistance due to temperature changes, it is not required that both strips have equal resistivity. (D) Coefficient of linear expansion - This is the property that causes metal strips to expand or contract differently due to temperature changes, making them ideal for usage in a thermostat.

Based on our understanding of the role of each property in the functioning of a thermostat, it is clear that the two metal strips must have different coefficients of linear expansion to ensure proper functioning. Therefore, the correct answer is: (D) Coefficient of linear expansion