What is activation energy?
A.It is the minimum amount of energy it takes to start a chemical reaction.
B.It is the maximum amount of energy it takes to start a chemical reaction.
C.It is the intermediate compound in a chemical reaction.
D.It is the energy needed for an exothermic reaction.
E.It is the energy needed for an endothermic reaction.

Hint: Activation energy is the energy that must be provided to compounds to result in a chemical reaction. The activation energy of a reaction is measured in joules per mole, kilojoules per mole, or kilocalories per mole.


Complete step by step solution:
Activation energy, in chemistry, is the minimum amount of energy which is required to activate atoms or molecules to such a condition in which they can undergo a chemical transformation or change or physical transport or movement. In transition-state theory, the activation energy is the difference in energy content between atoms or molecules which are in an activated or transition-state configuration and their corresponding atoms and molecules which are in their initial configuration. The activation energy is usually represented by the symbol ${E_a}$.
In mathematical expressions for such quantities as the reaction the rate constant is:
$k$= $A\exp ( - {E_a}/RT)$
and the diffusion coefficient,
$D$=${D_o}\exp ( - {E_a}/RT)$
Activation energies are resolved or calculated from trial rate constants or dissemination coefficients which are estimated at various temperatures.

So the correct answer is A.

Additional Information
Activation Energy can also be defined as the minimum amount of energy needed to energy required by reacting molecules to get converted into its product. It is generally measured in Joules and kilojoules per moles or kilocalories per mole.
Basically the activation energy is depend upon two factors that are:
1.The nature of the reactants is that if an ionic reactant is there then the activation energy will low. This is due to an attraction between reacting elements. And if the reactants are covalent then the value of activation energy will be high. This is because of the energy required to break the strong bond between them.
2.The effect of catalyst in the reaction. If a positive catalyst is added it will have low activation energy. However the negative catalyst will have a higher value of activation energy.

Note: In some cases, paces of response decline with expanding temperature. When following an around exponential relationship so the rate steady can at present be fit to an Arrhenius articulation, this outcomes in a negative estimation of Ea. Rudimentary responses displaying these negative initiation energies are regularly barrier-less responses, in which the response continuing depends on the catch of the particles in a likely well. Expanding the temperature prompts a diminished likelihood of the impacting atoms catching each other (with additionally looking crashes not prompting response as the higher force completes the impacting particles of the possible well), communicated as a response cross area that diminishes with expanding temperature.