Final answer:
The transition state represents the highest energy species in a reaction's energy diagram and is the peak that must be overcome by reactants to transform into products. Therefore, correct option is 3.
Step-by-step explanation:
In an energy diagram for a chemical reaction, which species represents the highest energy? The correct answer is the transition state or activated complex. This high-energy, unstable species is formed momentarily when reactant molecules collide with sufficient energy.
The energy hill, or activation energy, is the energy required for reactants to reach this state, and it is always positive, whether the reaction is exergonic (energy releasing) or endergonic (energy absorbing).
The transition state exists at a higher energy level than both the reactants and products, representing the peak of the energy diagram. By surpassing this activation energy barrier, reactant molecules can transform into products through bond-breaking and bond-making processes.