The 2nd illustration is the first to represent an equilibrium. 2 molecules of $A_2B$ reacted initially.
Work Step by Step
The second illustration shows the highest concentration of products (2 molecules) that the reaction will reach; therefore, the reaction has reached equilibrium. The first illustration shows two $A_2$ molecules and one $B_2$ molecule, and since the reaction started off with only $A_2B$ molecules, two $A_2B$ molecules must have reacted to produce those molecules.