Correct option is B
The Hammett equation describes a linear free-energy relationship relating reaction rates and equilibrium constants for many reactions involving benzoic acid derivatives with meta- and para- substituents to each other with just two parameters: a substituent constant and a reaction constant.
relating the equilibrium constant, K, for a given equilibrium reaction with substituent R and the reference constant K0 when R is a hydrogen atom to the substituent constant σ which depends only on the specific substituent R and the reaction rate constant ρ which depends only on the type of reaction but not on the substituent used.
The reaction constant, or sensitivity constant, ρ, describes the susceptibility of the reaction to substituents. Information on the reaction and the associated mechanism can be obtained based on the value obtained for ρ. If the value of: ρ<0, the reaction builds positive charge (or loses negative charge).
