The ratio of specific rate constants at two different temperatures, separated by 10 °C, is called as: 

CONCEPT:

Temperature Coefficient

• The temperature coefficient is the ratio of specific rate constants (k) at two different temperatures separated by 10°C.
• This is a measure of how the rate of a chemical reaction changes with temperature.
• It is based on the principle that reaction rates generally increase with temperature due to increased molecular collisions and the energy required to overcome the activation barrier.
• Mathematically, the temperature coefficient is expressed as:

  Temperature Coefficient = k₂/k₁

  Where k₂ and k₁ are the rate constants at two temperatures separated by 10°C.

EXPLANATION:

• The ratio of specific rate constants at two different temperatures is not related to activity coefficient, activation energy, or threshold energy.
• Activity coefficient measures the effective concentration of a species in a solution and is unrelated to temperature changes.
• Activation energy refers to the minimum energy required to start a reaction, while threshold energy is the minimum energy required for molecular collisions to result in a reaction.
• The specific term "temperature coefficient" directly relates to the change in reaction rate constants with temperature, making it the correct answer.

Therefore, the correct answer is option 4: Temperature Coefficient.