The given figure shows the potential energy curves for four...

The minimum of a potential energy curve indicates the equilibrium internuclear distance (bond length) on the x-axis and the bond dissociation energy (depth of the potential well) on the y-axis.

As the size of the halogen atom increases down the group ($F < Cl < Br < I$), the bond length of the corresponding hydrogen halide increases: $HF < HCl < HBr < HI$.

Conversely, as bond length increases, bond strength decreases. Thus, the bond dissociation energy follows the order: $HF > HCl > HBr > HI$.

Analyzing the curves:
Curve P has the shortest bond length and highest bond energy, so it represents $HF$ (A $\rightarrow$ II).
Curve Q has the second shortest bond length and second highest bond energy, representing $HCl$ (B $\rightarrow$ IV).
Curve R represents $HBr$ (C $\rightarrow$ I).
Curve S has the longest bond length and lowest bond energy, representing $HI$ (D $\rightarrow$ III).

The correct matching is A-II, B-IV, C-I, D-III.

Answer: A-II, B-IV, C-I, D-III