Question

In the PE diagram, which species is present in the lowest concentration? Enter a letter or a number.

Answer 1

Potential energy diagram represents relative energies of reactants, intermediates, transition state, and products.

In this PE diagram :

Reactant - A ; Intermediates - B,C ; Product - D

Main Transition states - 2,4,6

A transition state represents an energy maximum : any small displacement in it leads to
more stable product. It can never be isolated.
An intermediate is a molecule / ion which represents a localized energy minimum—an
energy barrier must be crossed before the intermediate goes to something more stable. An
intermediate can in principle be isolated (practically its high energy makes it difficult).

A species which has highest energy is least stable and presents species which is highly unstable, thus present in least concentration : as per this PE species 4   is present in the lowest concentration.