The reaction rate of a chemical reaction is determined by the slow step in the reaction mechanism.
The rate ii given by the product of the species of the determining step, each raised to its coeeficient in the chemical reaction that represents this step.
So, given that the determining step is represented by A + 2B→C, the reaction rate is r = k[A]*[B]^2.
This mechanism is verified becasue the total order is the summ of the coefficientes : 1 + 2 = 3 and the exponent 2 makes B is a bigger rate determinant than A.
So, the answer is the option b. k[A][B]^2.