The temperature coefficient of a reaction is defined as the ratio of the rate constant at a temperature ten degrees higher to the rate constant at the lower temperature, usually expressed as k at (T+10) divided by k at T. For many reactions near room temperature this ratio lies between two and three, which is the origin of the common observation that the rate roughly doubles for each ten-degree rise. This empirical quantity reflects the strong, exponential dependence of the rate constant on temperature described by the Arrhenius equation. The option order of reaction concerns concentration dependence, not temperature. Activation energy is the energy barrier itself, not the ratio of rate constants. The rate constant is a single value at one temperature, not a ratio. The temperature coefficient gives a quick experimental sense of how temperature-sensitive a reaction is, as discussed in NCERT. Plausibility check: a ratio close to two for a ten-degree rise matches the typical activation energies of common reactions, confirming the definition.