To solve this problem, we need to analyze the given assertion and reason.$\newline$Assertion A: A reaction can have zero activation energy.$\newline$Reason R: The minimum extra amount of energy absorbed by reactant molecules so that their energy becomes equal to threshold value is called activation energy.$\newline$Let's evaluate each statement:$\newline •$Activation energy is the energy barrier that must be overcome for a reaction to proceed.$\newline •$In some cases, particularly in reactions involving radicals or certain catalyzed reactions, the activation energy can be very low or even zero.$\newline •$Therefore, Assertion A is true.$\newline •$Reason R correctly defines activation energy as the minimum extra energy required for reactants to reach the threshold energy.$\newline •$Thus, Reason R is also true.$\newline$However, Reason R does not explain why a reaction can have zero activation energy.$\newline$The reason a reaction can have zero activation energy is due to specific reaction mechanisms or the presence of catalysts, which is not explained by R.$\newline$Therefore, the correct answer is:$\newline$Option 4: A is false but R is true.